Metal gasket

ABSTRACT

A metallic gasket including a base plate having a convex metal bead formed, by bending the base plate, between a combustion chamber opening and on the outer periphery portion of the base plate, and a first thickness-increased portion formed thicker than the remaining portions of the base plate to provide a difference in thickness, by which a surface pressure is made to concentrate on the first thickness-increased portion to seal a joint between a cylinder head and a cylinder block when the metallic gasket is disposed between the joint surfaces of the cylinder head and the cylinder block and fastened by clamping bolts. An elastic sealing material is fixed to the convex portion of the metal bead and filled in a concave portion on the reverse side of the base plate to form a rubber bead, and the sealing material is deformed under compression in the through-thickness direction in cooperation with the deformation of the metal bead when the gasket is fastened with clamping bolts. This gasket reduces production cost and preserves a stable sealing performance for extended periods of time.

TECHNICAL FIELD

[0001] The present invention relates to a metallic gasket sandwichedbetween opposing joint surfaces, and more particularly to a metallicgasket to seal joint surfaces of a cylinder block and a cylinder head ofan internal combustion engine, for example, by being sandwiched betweenthe joint surfaces.

BACKGROUND ART

[0002] Among conventional metallic gaskets of this kind, there is ametallic gasket shown in FIG. 51 that is described in Japanese PatentLaid-Open No. 6-101761, for example. More specifically, this well-knownmetallic gasket is provided with a thickness-increased portion X formedat an inner peripheral portion of the combustion chamber opening 2 in abase plate 1 by folding back the peripheral portion on the combustionchamber opening 2 side of the base plate 1 or by mounting a shim plate 3to the outer peripheral portion of the base plate and this gasket isalso provided with a second thickness-increased portion C, which isthinner than the first thickness-increased portion X and formed byfolding back an outer peripheral portion of the base plate. In addition,a rubber bead 4 is fixed on the outer side of the firstthickness-increased portion X in a manner to protrude on both surfacesof the base plate and a rubber bead 5 lower in thickness than thethickness of the rubber bead 4 is fixed on the inner side of the secondthickness-increased portion C in a manner to protrude on both surfacesof the base plate.

[0003] When the metallic gasket structured as described is sandwichedbetween the joint surfaces of the cylinder block and the cylinder headand fastened by clamping bolts, the rubber beads 4, 5 are compressed anddeformed in the through-thickness direction, and at the end offastening, because of the thickness difference between the firstthickness-increased portion X of the largest thickness and the remainingportions of the base plate, surface pressure concentrates on the firstthickness-increased portion and the largest load acts on it.

[0004] Therefore, applied to the joint is a fourfold seal which includesthe surface pressure of the first thickness-increased portion X and theresilience of the rubber bead 4, and the surface pressure of the secondthickness-increased portion C and the resilience of the rubber bead 5.Further, owing to the thickness-increase effect of the firstthickness-increased portion X, the rubber bead 4 is prevented fromtotally collapsing and, also owing to the thickness-increase effect ofthe second thickness-increased portion C, the rubber bead 5 is preventedfrom totally collapsing.

[0005] As a second conventional metallic gasket, there is one which isdescribed in Japanese Utility Model Application Laid-Open No. 63-180769,for example.

[0006] This metallic gasket is made by forming a base plate from a thinmetal plate having rigidity, such as stainless steel plate, and variouskinds of holes are formed in the base plate, such as an opening for thecombustion chamber bore, bolt holes, oil holes. The holes, which requiresealing, are enclosed by a full bead provided along the seal line. Arepeated load by vibration amplitude caused by engine operation isapplied to the metallic gasket, and in order to prevent fatigue failureof the bead by the repeated stress, the thickness-increased portions,formed to suppress the amount of bead deformation that occurs in thethrough-thickness direction, are provided along the whole internalperipheral portion on the combustion chamber opening side of the baseplate.

[0007] With the progressive reductions of weight and size of the engine,the clearance between the adjacent combustion chamber openings isbecoming smaller and the range in which beads and thickness-increasedportions can be provided is becoming narrower in the boundary areabetween the adjacent combustion chamber openings. In view of this trend,in the prior art, a shared bead has come to be used in the boundaryarea, and furthermore, a thickness-increased portion with a minimumwidth is formed between the combustion chamber openings in the middle ofthis boundary area and a wide-width thickness-increased portion isformed in each end of the boundary area according to the clearancebetween the peripheral edges of the adjacent combustion chamberopenings.

[0008] In order to secure a necessary sealing pressure by the bead whichis elastically deformed in the through-thickness direction, ahigh-rigidity material, as mentioned above, is used for the conventionalbase plate.

[0009] Moreover, as the third conventional kind of metallic gasket,there is one which is disclosed in Japanese Patent Laid-Open No.63-210465, for example.

[0010] In this metallic gasket, bead is a metal bead that is formed bybending the base plate (Refer to FIG. 52), and as shown in FIGS. 53 and54, in a converge-diverge points M where seal lines 50 converge anddiverge, the bead width becomes relatively wide, and after the seallines 50 come together, they merge into a seal line 50 and the seal line50 gradually becomes narrower.

[0011] There is another form of converge-diverge point which isdescribed in Japanese Patent Laid-Open No. Hei 1-300043. Morespecifically, in this example, as shown in FIG. 55, a full bead which isprotruded in one surface side in the form of mountain branches off intotwo half beads in stepped form, or, viewed differently, the two halfbeads converge into a full bead. This is a metal bead formed by bendinga base plate.

[0012] Further, the fourth conventional type of metallic gasket isdisclosed in Japanese Patent Laid-Open 2001-173791.

[0013] As shown in FIG. 59, this metallic gasket is formed by two baseplates 50. More specifically, out of the two base plates 50, in thethicker base plate (the upper base plate), its end portion on thecombustion chamber opening 51 side is bent to form a thickness-increasedportion 52, and in each of the base plates 50, a convex bead 53 isformed on the outer side of the thickness-increased portion 52 so as tobe higher than the thickness of the thickness-increased portion 52, andboth base plates 50 are put together such that the convex portions ofthe base plates 53 facing each other. Moreover, the concave portions ofthe beads each facing outside are filled with an elastic sealingmaterial 54.

[0014] When the above-described metallic gasket is sandwiched betweenthe opposing joint surfaces of the cylinder block and the cylinder headand they are fastened by clamping bolts, the base-plate beads arecompressed and deformed down to the thickness of the thickness-increasedportion in the inner peripheral portion of the combustion chamberopening, and the elastic sealing material 54 filled in the concaveportions is compressed and deformed to thereby seal the combustion gas,oil and cooling water pressure by a sealing pressure generated by acomposite force including the spring force of the beads 53 made of thebase plate and the resilience of the elastic sealing material 54. Itgoes without saying that there are conventional metallic gaskets withoutany elastic sealing material 53 filled in the concave portion and alsothere are conventional metallic beads made up of a single piece of baseplate.

[0015] However, in the first conventional metallic gasket shown in FIG.51, the rubber beads 4, 5 are provided on both surfaces of the baseplate 1, and if the thickness of the folded-back portion of thethickness-increased portion is designated as t0, because the rubberbeads 4 are provided on each surface of the base plate 1, the height ofa rubber bead 4, for example, is t0/2+the compression-deformed amount((t0/2)

0.4 (40% max.)). Assuming that t0 is 0.5 mm, the height of the bead 4 is0.35 mm from the above equation. Thus, if the thickness of the baseplate is thicker, it is possible to set a compression-deformed amountwhich sufficiently suits processing accuracy. However, if the thicknessof the base plate is thinner, the compression-deformed amount is smallso that it becomes difficult to process rubber beads, processingaccuracy becomes stricter, thus increasing production cost.

[0016] Since water holes are normally formed in the base plate 1 betweenthe rubber beads 4, 5, the rubber beads 4, 5 are exposed to coolingwater, thus deteriorating their durability.

[0017] In the second conventional metallic gasket, a high-rigidity thinmetal plate is adopted for the base plate to obtain a sufficient sealingpressure by elastic deformation of the beads, and therefore athickness-increased structure is adopted to prevent fatigue failure ofthe beads, which is likely to occur by adoption of thin metal plate ofhigh rigidity. However, the engines do not cease evolving and attemptshave been made to reduce engine size and weight, improve performance anddecrease fuel consumption, and as a result, there is a tendency towardhigher combustion temperature and larger amplitude of vibration.

[0018] Therefore, even if a thickness-increased structure is adopted toprevent fatigue failure of the bead, as long as the conventionalmetallic gasket structures is used, there is a possibility that the beadsuffers fatigue failure in a period of time shorter than a plannedlifetime.

[0019] In the engines, a changeover to aluminum has been taking placefor lightweight and better workability. When a cylinder block and acylinder head are made by casting of aluminum, blowholes occur in thecasting process, and while the joint surfaces are machined, blowholesmay run through one to another, stretching across the seal line of thebead, giving rise to incomplete sealing somewhere in the joint.

[0020] Further, during plane processing of the joint surfaces of theengine, tool marks are left on the surfaces. To compensate for the toolmarks, heat-resistant rubber with a thickness about twice the depth ofthe tool marks is applied to the gasket in a manner to cover the wholesurface of the base plate with a rubber coating. If processing finish isrough, it is necessary to increase the thickness of the rubber coating,which will lead to a decrease in torque.

[0021] As has been described, the bead surfaces are covered with a thinrubber coating and the bead is so formed as to seal the joint by itsrounded portion.

[0022] Since the bead is formed by thin metal plate with high rigidity,it has a high resilience, and though the thickness-increase effect hasbeen reduced, because internal stress concentrates in theabove-mentioned rounded portion, when fatigue failure runs from onecrack to another or the rubber coating is abraded off by vibrationamplitude, the metal at the rounded portion of the gasket directlycontacts the seal surface, which results in fretting or gas leak orfatigue failure.

[0023] Further, since the engines have been reduced in size and weight,the rigidity of the engines has decreased, unless the fastening axialtension is decreased, bore deformation will increase, which leads togreater oil consumption or power loss. For this reason, it appears thatthere is high demand for metallic gaskets which maintain sufficientsealing performance even if the fastening axial tension is made smallerthan before.

[0024] In the third conventional metallic gasket, the bead seals thejoint by metal contact and line contact by using a metal bead.Therefore, it is necessary to form the base plate, in other words, thebead by a material of high rigidity to increase the spring stress of thebead to secure a required sealing pressure.

[0025] More specifically, as shown in FIGS. 53, 54, it is inevitablethat there are some partially wide parts at the converge-diverge pointsM of the seal line 50. As described above, this gasket has a structureto seal the joint only by the spring force of the metallic bead, andforms a line seal at three points (See A, B and C in FIG. 52) of theradius R of the bead, and at the converge-diverge point M where the beadwidth widens, as the span A-B becomes wide, the spring force becomesweak and sealing surface pressure becomes relatively low. However, it isimpossible to increase the outer side radius R of the converge-divergepoints M of the seal line 50.

[0026] Around the converge-diverge point M of the seal line 50, the beadhas a relatively narrow width and forms a small radius R, with theresult that the bead has a strong spring force close to that of a rigidbody. Near the bead with strong spring force, there is locally formed awide-width bead with a small spring force, in other words, partly with alow sealing pressure, and when a high pressure is applied to thewide-width bead, the bead is deformed partly, which results in leakageof pressure or liquid.

[0027] A metallic gasket with the above problem in mind is shown inFIGS. 56 and 57. With the bead of this metallic gasket, each of the seallines that flow together into the converge-diverge point M is designedto keep its width as constant as possible. However, as described above,a high hardness material is used to provide high rigidity by which togenerate a high spring force, for which reason the converging radius Rof the line outside the converge-diverge point M cannot be made large,so that this gasket is nothing other than a minor improvement that isunable to obtain an equalized surface pressure because theabove-mentioned radius R is small.

[0028] Among the conventional metallic gaskets is a type disclosed inJapanese Utility Model Laid-Open No. 5-42830. As shown in FIG. 58, thismetallic gasket has a concave space formed in the middle of a wide-widthconverge-diverge point M to keep the bead width of the converge-divergepoint M as constant as possible. However, this gasket has a shortcomingthat the concave space in the middle is confined by seal lines,resulting in the converge-diverge point M having too strong a springforce.

[0029] As has been described, at the converge-diverge point M of theseal line 50, the bead width of the metal bead changes locally, and itis difficult to keep a uniform bead width. With the above-mentionedconventional metallic gasket, because it is necessary to set a largespring force for the bead, a high hardness material is used. Therefore,if the bead width changes as mentioned above, the spring force changesgreatly. With the structure that parts of small radius R are inevitablyformed as in the converge-diverge point M and that some parts existwhere the spring force is large locally, supposing that the engine ismade of aluminum, the sealing surfaces of the engine are prone todenting and scratching. If the engine sealing surfaces have local dentedflaws, the flaws will lead to pressure leak when the gasket is changed.Even if the engine is made of cast iron, the converge-diverge point M isformed in a structure such that the surface pressure of theconverge-diverge point M of the seal line 50 is high at the part ofsmall radius R of the line outside the part M and that the part of alarge radius R inside the part M is liable to oil leakage.

[0030] A base plate that has a larger number of converge-diverge pointsM requires a larger fastening force, and thus requires a larger totalfastening axial tension.

[0031] When the rigidity of the region of the outer periphery of thebase plate is weak, because of a large stress at the converge-divergepoint M, the region of the outer periphery of the base plate is subjectto a large deformation, which may lead to pressure leakage.

[0032] Besides on the converge-diverge part M, internal stressconcentrates on the rounded portion of high hardness material, which maylead to fatigue failure by the amplitude of vibration and further leadto short lifetime.

[0033] In the fourth conventional metallic gasket (See FIG. 59), aresilience is generated jointly by the base-plate beads 53 and theelastic sealing material 54 filled in the concave portions when they aredeformed by fastening, thus generating a required sealing pressure alongthe seal line.

[0034] However, if the base plates 50 are formed by metal plate of lowhardness with a view to preventing fatigue failure of the base-platebeads 53 and reducing cost, in the above-mentioned conventional metallicgaskets, when bolts are fastened and the elastic sealing material 54 ofthe concave portion of the bead is compressed and deformed, an externalforce is applied such that the base plates 50 and the base-plate beads53 are deformed in a manner to warp in the through-thickness direction.Since the base plates 50 are formed of metal of low hardness asmentioned above, the beads have a low shape-retaining force andaccordingly the base plate 50 have an insufficientdeformation-preventive force and hence a low sealing property.

[0035] By repeated load by repetition of operation and stoppage of theengine, after a long period of use, problems arise, such as a decreasein axial tension of the clamping bolts, changes with time of thebase-plate bead 53 on the base plate 50, or deterioration in the elasticsealing material 54 of the concave portion of the bead; therefore, thesealing surface pressure is likely to drop. Such problems tend tomanifest particularly at overhanging parts on the outer side of theclamping bolts.

[0036] When the elastic sealing material 54 is formed by baking in theconcave portions of the base plates, even if the elastic sealingmaterial 54 at high temperature is filled in the concave portions, itchanges in volume by an amount of thermal expansion during subsequentopen cooling, the center portion of the elastic sealing material 54where the thickness is at its highest shrinks by an amount of thermalshrinkage. This is disadvantageous when the surface pressure decreasesas described above. Such a phenomenon as this seems to be likely tooccur particularly when the gasket is mounted in the engine which hasbeen assembled with a weak fastening axial tension.

SUMMARY OF THE INVENTION

[0037] The present invention has been made to solve the above problems,and has as its task to provide a metallic gasket capable of reducingcost and securing a stable seal performance for a long period of time.

[0038] To achieve the above task, a metallic gasket comprising a baseplate made of a thin metal plate and having formed therein one or notless than two openings, such as bolt holes, beads formed along seallines and a first thickness-increased portion thicker than the remainingportions of the base plate to cause a surface pressure to concentrate onthe first thickness-increased portion and said bead to be deformed inthe through-thickness direction to thereby seal a joint of opposingjoint surfaces when the metallic gasket is sandwiched between the jointsurfaces and fastened by clamping bolts,

[0039] wherein said bead comprises, in combination, a metal bead and arubber bead, the metal bead being formed by bending the base plate inthe through-thickness direction to form a convex portion, on one surfaceof the base plate, with a height higher than the thickness of the firstthickness-increased portion, and the rubber bead made of an elasticsealing material being fixed to a surface of the convex portion of themetal bead and filled in a concave portion opposite the convex portion,wherein the rubber bead is compressed and deformed in thethrough-thickness direction in cooperation with the deformation of themetal bead.

[0040] The first thickness-increased serves to limit thecompression-deformed amount of the bead to an appropriate value, inother words, can securely prevent compression fracture of the bead.

[0041] The amount of the elastic sealing material, which has beenincreased, on the convex portion side of the metal bead increases thecompression-deformed amount, facilitating processing of an elasticsealing material and making it possible to increase the thickness of theelastic sealing material. Therefore, it becomes possible to set aprocessing tolerance on the large side, and reduce manufacturing cost.

[0042] Moreover, this metallic gasket offers less opportunity for theelastic sealing material on the convex portion of the metal bead to beexposed to cooling water and the elastic sealing material filled in theconcave portion of the metal bead is covered with the metal bead and isnot exposed to cooling water. Thus, preventing the elastic sealingmaterial from deteriorating in quality and securing a stable sealperformance for a long period.

[0043] Further, a necessary sealing pressure can be obtained by synergyof the resilience of the metal bead and the elastic resilience of therubber bead made by an elastic sealing material fixed to the convexportion and filled in the concave portion of the metal bead, so that itbecomes possible to decrease the hardness of the base plate and stuntthe fatigue failure of the metal bead. Moreover, it is possible toabsorb the vibration amplitude of the engine and also absorb theroughness of the seal surface, and it is possible to appropriately sealoff cooling water pressure and oil pressure with not large pressure.

[0044] Further, a wide sealing area can be taken for the elastic sealingmaterial on the convex portion of the metal bead and also for theelastic sealing material on the concave portion of the metal bead.Therefore, it is possible to suitably seal the flaws on the jointsurface and blowholes, which occur in casting, with a low surfacepressure. Moreover, since the elastic sealing material consists of anelastic substance (rubber-based, above all else), the gasket factor islow, for which reason a limited axial load can be utilized effectivelyin the areas under adverse condition, thereby reducing total load.

[0045] A case is assumable where an elastic sealing material is usedonly in the concave portion, but if the base plate has a low hardness,when the elastic sealing material in the concave portion is compressedand deformed, there is a possibility that the metal bead and the flatportion of the base plate continuous to the metal bead are deformed inthe through-thickness direction. In this case, the sealing pressure isdecreased by a deformation in the through-thickness direction. Incontrast, in the present invention, an elastic sealing material isapplied even on the side where there is the convex portion and by thiselastic sealing material on the convex portion side, the above mentioneddeformation in the through-thickness direction is inhibited, therebyinhibiting a decrease in the sealing pressure.

[0046] Incidentally, it is desirable to limit the largest width of therubber bead on the convex portion side within 1.5 times the width of themetal bead with the exception of the portions which are subjected to alarge pressure such as bolted portions. It is necessary to apply moreload to the base plate where the rubber bead has a larger width. From aviewpoint of inhibiting an increase in load, it is desirable to limitthe width of the bead of an elastic sealing material within 1.5 timesthe width of the metal bead as mentioned above. This does not apply towhere one wants to apply large load locally.

[0047] Next, the invention is characterized in that the metal bead iseither a full bead or a half bead in a stepped form.

[0048] When a bead protrudes on one surface from a reference plane ofthe base plate, this is a convex bead portion of the bead. On thesurface of the convex portion surface, a concave portion is necessarilyformed. Note that with a half bead mentioned above, the concave portionon the reverse side of the surface of the convex portion forms aninclined surface, and the elastic seal may be fixed to a flat placecontinuous to the inclined surface.

[0049] The invention is also characterized in that the elastic sealingmaterial fixed to the surface on the convex side of the metal bead isfixed at least to the surface of convex portion.

[0050] The elastic sealing material is fixed to the convex portion ofthe metal bead, in other words, the surface rising from the base-plateflat surface, such as the inclined surface, thereby leaving less chancesfor the elastic sealing material to separate from the base plate, andeven when a rubber bead is formed, the above-mentioned structureprevents the bead width from becoming wider than required.

[0051] Next, the invention is also characterized in that the height ofthe elastic sealing material fixed to the surface of the convex portionof the metal bead is equal to or substantially equal to the convexportion of the metal bead.

[0052] The invention is also characterized in that some of openingsformed in the base plate are combustion chamber openings and that theabove-mentioned first thickness-increased portion is formed in a mannerto endlessly encircle the combustion chamber openings.

[0053] By providing the first thickness-increased portion at theperipheral edge of the combustion chamber opening, it becomes possibleto seal the combustion chamber opening at a high pressure.

[0054] The invention is also characterized in that a peripheral endportion on the combustion chamber opening side of the base plate isfolded back and the first thickness-increased portion is formed byputting a shim plate inside the folded-back portion.

[0055] The total thickness of the first thickness-increased portion ismade thin in the vicinity of a bolt hole where the surface pressure isrelatively high, and it is made thick between bolt holes where thesurface pressure is relatively low. Therefore, the surface pressure thatacts on the first thickness-increased portion is equalized in thecircumferential direction when clamping bolts are fastened, so that theroundness of the cylinder bore can be secured, and the axial tension ofthe clamping bolts can be reduced, thereby properly preventing theengine, particularly a low-rigidity engine from being deformed.

[0056] The invention is also characterized in that the total thicknessof the first thickness-increased portion is made thin in the vicinity ofthe holes for clamping bolts and is made thick between thebolt-receiving boles and the total thickness is varied in the radialdirection of the combustion chamber openings.

[0057] Next, the invention is characterized in that the secondthickness-increased portion thinner than the first thickness-increasedportion is provided in the region along the outer periphery of the baseplate.

[0058] The invention is also characterized in that lubricant is appliedto one or both surfaces of the base plate.

[0059] By application of lubricant to one surface or two surfaces,fretting can be prevented which is caused by shifts or knockingresulting from a difference in thermal expansion between the engine andthe gasket or from vibration amplitude. In other words, lubricant shoulddesirably be applied the parts that contact the joint surfaces.

[0060] In the invention, a metallic gasket comprising a base plate madeof a thin metal plate having formed therein a plurality of combustionchamber openings arranged adjacent to each other, a thickness-increasedportion thicker than the remaining portions of the base plate and seallines formed encircling the internal circumference of each combustionchamber opening, and beads formed along the seal lines,

[0061] wherein in a boundary area between the adjacent combustionchamber openings, common beads are formed to be shared by the adjacentcombustion chamber openings, and beads formed along the neighboringperipheral edges of the adjacent combustion chamber openings arecombined to form a bead in an integral structure, wherein the widths ofthe thickness-increased portions located in the boundary area andextending along the internal circumferences of the combustion chamberopenings are set according to a clearance between the adjacentcombustion chamber openings, wherein the bead is a composite body of ametal bead formed as a convex portion having a height higher than thethickness-increased portion by bending the base plate in thethrough-thickness direction and a rubber bead formed by fixing to theconvex portion surface of the metal bead an elastic sealing materialcapable of exhibiting a spring force by deformation under compression inthe through-thickness direction, and by filling the concave portion onthe reverse side of the convex portion with the elastic sealingmaterial, and wherein the elastic sealing material is fixed at least tothe surface of the convex portion of the metal bead and the height ofthe elastic sealing material is set to be equal to or substantiallyequal to the height of the metal bead.

[0062] In the boundary area between the adjacent combustion chamberopenings, the clearance along the internal circumferences of thecombustion chamber openings is normally smallest at the middle point ofthe boundary area, so that the width of the thickness-increased portionis narrowest at the middle point and becomes wider toward both ends.

[0063] According to this invention, the required sealing pressure isgenerated by synergy of the resilience of the metal bead and the elasticresilience of the elastic sealing material fixed to the metal beadposition, and because of this the hardness of the base plate materialcan be reduced. Therefore, the internal stress locked up in the foldedportion of the radius R is so small that there is no room for worryabout fatigue failure. Moreover, because of the improved workability informing the thickness-increased portion, use of soft base plate for themetal bead, and use of elastic, flexible rubber bead (elastic sealingmaterial) attached to both surfaces of the metal bead, the deformationload of the bead has been reduced drastically and most of the fasteningaxial tension can be applied to the thickness-increased portionencircling the combustion chamber opening, making it possible todecrease the fastening axial tension.

[0064] Since a large seal area can be secured which is covered with anelastic sealing material, the flaws on the joint surface and blowholesthat occur in casting can be sealed properly with a low surfacepressure.

[0065] Because an elastic sealing material is fixed to the surface ofthe convex-side base plate of the metal bead where a gap would occurbetween the joint surface and the base plate, more specifically, fixed,for example, to the position that faces the external circumferentialregion of the joint surfaces, entry of water or the like into the gapcan be prevented.

[0066] In addition, by fixing the elastic sealing material to the convexportion, in other words, to the inclined surface of the metal bead, theelastic sealing material can be made less likely to separate from thebase plate.

[0067] Next, the invention is characterized in that thethickness-increased portion is varied in thickness partly to equalizethe surface pressure when the gasket is inserted between the jointsurfaces.

[0068] For example, the thickness of the thickness-increased portion inthe vicinity of the bolt holes or the like is set to a relatively lowlevel, where the surface pressure is likely to be high.

[0069] According to the present invention, because the boundary areabetween the adjacent combustion chamber openings is narrow, even if thewidth of the thickness-increased portion along the internalcircumference of the combustion chamber opening changes, in other words,even if the thickness-increased portion has a minimum width in themiddle part of the boundary area, for example, the thickness of thethickness-increased part is varied in the region along the circumferenceto thereby equalize the surface pressure during fastening along thecircumference in order that an effective sealing performance can beobtained even in the narrow-width thickness-increased portion betweenthe combustion chamber openings.

[0070] At this time, as has been described above, because the hardnessof material for the base plate can be decreased, it becomes easy tolocally change the height of the thickness-increased portion.

[0071] Next, the invention is also characterized in that at least theprotrusion height or the width of the metal bead is varied partly alongthe seal line to thereby equalize the sealing surface pressure by thebead provided along the seal line.

[0072] Next, the invention is characterized in that at least theprotrusion height or the width of the rubber bead is varied partly alongthe seal line to thereby equalize the sealing surface pressure by thebead provided along the seal line.

[0073] Next, the invention is characterized in that a metallic gasketincludes a base plate having a plurality of seal lines formed on thebase plate made of thin metal plate, beads formed along each seal line,and a converge-diverge part formed where at least some of the pluralityof seal lines converge or diverge at specified points,

[0074] wherein the bead formed along each seal line comprises, incombination, a metal bead and a rubber bead, the metal bead being formedin a convex shape only on one surface of a base plate by bending in athrough-thickness direction thereof and the rubber bead made of anelastic sealing material being fixed to a surface of the convex portionof the metal bead and filled in a concave portion on the reverse side ofthe convex portion, the rubber bead being compressed and deformed in thethrough-thickness direction in cooperation with the deformation of themetal bead, and wherein at least in the converge-diverge part, theelastic sealing material on the surface of the convex portion is fixedat least to the surface of the metal bead and the height of the rubberbead is set to be equal to or substantially equal to the height of themetal bead.

[0075] According to this invention, the bead is formed in a compositestructure of a metal bead and a rubber bead to make it possible todecrease the hardness of the metal bead, in other words, the base plate.As the hardness of the metal bead is decreased, the difference insealing pressure caused by a difference in the width of the metal beadis substantially reduced.

[0076] The invention is characterized in that a thickness-increasedportion is formed by partly increasing the thickness of the base platein the through-thickness direction, and the compression-deformed amountof the bead is regulated by the increased thickness of thethickness-increased portion.

[0077] In the wide-width portion formed in the converge-diverge part, aslong as the seal function is achieved by the elastic sealing materialfilled in the concave portion of the bead and by the elastic sealingmaterial part formed on the convex portion side, since the metal bead isformed in the base plate of low hardness and a spring load is generatedby a soft elastic sealing material attached to both surfaces of themetal bead, the bead according to this invention tends to be weaker thana spring force generated by a base plate of high hardness, and if anexcessive load is applied owing to a large volume expansion or if theamplitude of vibration is great, for example, or if the fastening axialtension is large, for one thing, deformation will occur, which may leadto a decrease in a sealing force.

[0078] In contrast to this, in the invention set forth in claim 15, toprevent the above problem, a thickness-increased portion is formed byfolding back the whole inner peripheral edge of an opening in the baseplate or partly forming a thickness-increased portion by folding back apart of the outer peripheral edge of the base plate or creatingthickness-increased portions on both peripheral edges at the same time,this thickness-increased portion regulates the amounts of deformation inboth the metal bead in the base plate and the rubber bead on the upperand lower surfaces of the metal bead. In other words, the maximum amountof deformation is limited to a predetermined value even if the rigidityof the base plate is reduced, and the thickness-increased portionsuppresses excessive load or vibration amplitude, thereby securing astable sealing property for a long period of time. Moreover, even ifsoft steel is used as the base plate to provide less expensive gaskets,it is possible to provide metallic gaskets capable of a desired sealingof the joint.

[0079] When a thickness-increased portion is provided in the regionencircling the combustion chamber opening, by changing the amount ofthickness increase in the circumferential direction, it is possible toeven out the fastening surface pressure in the circumferentialdirection.

[0080] The invention is also characterized in that by adjusting thewidth of the rubber bead in the converge-diverge part, the sealingpressure in the converge-diverge part is made to match or come close tothe sealing pressure at the seal lines other than the converge-divergepart.

[0081] According to this invention, by adjusting the width of the rubberbead whose width at the converge-diverge part is easy to adjust, thesealing pressure of the converge-diverge part is adjusted so as to beequal to that of the other seal lines, making it possible to efficientlyrepress leakage of the seal.

[0082] Supposing a case where an elastic sealing material is appliedonly to the concave portion of the metal bead under the condition thatthe hardness of the base plate has been reduced to minimize thedeformation load in the metal bead and load on parts such as theconverge-diverge part, because the rigidity of the base plate is low,the metal bead is made liable to deform by the deformation of theelastic sealing material in the concave portion (the metal bead isdeformed in such a manner that that portion of the base plate lying onboth sides of the metal bead warps toward the convex portion side. Thisleads to deformation of the base plate itself.) Consequently, theabove-mentioned deformation further weakens the spring of the metalbead. In this invention, the spring force of the rubber bead isreinforced by providing the elastic sealing material on the convexportion side of the metal bead while inhibiting the lowering of thespring force by the above-mentioned deformation of the metal bead.

[0083] The largest width of the rubber bead provided on the convexportion side should preferably be within 1.5 times the width of themetal bead with the exception of the portions which are subjected to alarger pressure such as bolted portions. Because it is necessary toapply more load to the base plate where the rubber bead has a largerwidth, from a viewpoint of inhibiting an increase in load, it isdesirable to limit the width of the bead made of an elastic sealingmaterial within 1.5 times the width of the metal bead as describedabove. This does not apply where one wants to apply large load locally.

[0084] As the elastic sealing material provided on both surfaces of thebase plate makes area contact with the joint surfaces, an area seal isobtained, and furthermore a complete seal can be achieved when theprocessing roughness on the seal surfaces is absorbed and blowholes ascasting defects are within the bead width and the flexible elasticsealing material adapts itself to the flaws that run across the sealline, although the seal line used to have weakness for flaws along theouter periphery of the base plate, which has been a defect in theexisting linear seal line. Since an elastic sealing material partsuch asrubber, is used for the rubber bead, the gasket factor of the beadaccording to this invention is low, and if the gasket factor is not lessthan 2, a sufficient seal can be obtained, and as a result, totalfastening load can be reduced. Moreover, it becomes possible to adopt alow hardness material for the base plate, and because of this it ispossible to inhibit fatigue failure of the metal bead.

[0085] Next, the invention is characterized in that in theconverge-diverge part where the metal bead diverges from a full beadinto a plurality of full beads or a plurality of full beads convergeinto a full bead, the rubber bead width is adjusted so that the ratio ofthe rubber bead width on the convex portion side of the metal bead tothe metal bead width becomes smaller in the converge-diverge part thanin the remaining portions of the bead.

[0086] At a converge-diverge part where the seal lines intersect in theform of the letter T, Y or X, there is a part where the width of themetal bead is wide, and in this wide-width part, the concave portion ofthe metal bead is filled with a larger amount of elastic sealingmaterial than in the regular portion of the bead, which results in alarge deformation load. In contrast, in this invention, in order to makethe deformation load close to that of the bead other than at theconverge-diverge part, in the above-mentioned ratio, the width of therubber bead on the convex portion is formed to be narrower than thewidth of the regular portion of the bead to thereby inhibit an increasein the spring load in the wide-width portion at the converge-divergepart, thus evening out the sealing pressure along the seal line,including the converge-diverge part.

[0087] The invention is also characterized in that in theconverge-diverge part where the metal bead diverges from a full beadinto a plurality of half beads in a stepped form or a plurality of halfbeads in a stepped form converge into a full bead, the rubber bead widthis adjusted so that in the vicinity of the clamping bolts the ratio ofthe rubber bead width to the metal bead width becomes larger in theconverge-diverge part than in the remaining portions of the bead.

[0088] In the full bead where the metal bead is in a convex protrusionin the form of mountain, when deformed in the through-thicknessdirection, the elastic sealing material filled in the concave portion,being unable to escape, exhibits a sufficient spring force. In the metalhalf bead in a stepped form, the elastic sealing material filled in theconcave portion changes its shape as if to escape sideways when deformedin the through-thickness direction and its spring force becomes smaller.At the converge-diverge part, as the metal bead width becomes wider, thespring force becomes smaller. In this respect, by adjusting the rubberbead to make its width relatively wider, the sealing pressure along theseal line, including the converge-diverge part, is equalized.

[0089] Next, the invention is characterized in that one or not less thantwo lines of protrusion are formed along the seal line at least one ofthe surface of the elastic sealing material fixed to the surface of theconvex portion and the surface of the elastic sealing material filled inthe concave portion, in which those elastic sealing material partsconstitute the rubber bead.

[0090] Next, the invention is characterized in that one or not less thantwo lines of protrusion are formed on the portions where the sealingsurface pressure is relatively low on at least one of the surface of theelastic sealing material fixed to the convex portion and the surface ofthe elastic sealing material filled in the concave portion, thoseelastic sealing material parts constituting the rubber bead.

[0091] Next, the invention is characterized in that at least one of theheight and the width of each line of the above-mentioned protrusion isvaried in their extending direction according to the sealing surfacepressure at a position where the protrusion is formed and a larger valueof at least one of the height and the width of the protrusions is setwhere the sealing surface pressure is lower.

[0092] The invention is also characterized in that a plurality of rowsof protrusion are formed on at least one of portions of elastic sealingmaterial, either the one fixed to the surface on the convex portion orthe other one filled in the concave portion, and in the plurality ofrows of protrusion, at least one of the height of the protrusion and anarea per unit length is varied according to the sealing surface pressureat the position where the protrusion is formed.

[0093] Note that a protrusion is formed on the overhanging portion ofthe base plate where the surface pressure is weak, and that a protrusionmay not be formed in the vicinity of bolts.

[0094] The invention is also characterized in that a plurality of baseplates are stacked in multiple layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0095]FIG. 1 is a plan view of an essential part for explaining ametallic gasket according to a first embodiment of the presentinvention;

[0096]FIG. 2 is a sectional view taken along the line B-B in FIGS. 1 and13;

[0097]FIG. 3 is a sectional view taken along the line A-A in FIG. 1;

[0098]FIG. 4 is an explanatory sectional view for explaining anothermetallic gasket according to the first embodiment of the presentinvention;

[0099]FIG. 5 is an explanatory sectional view for explaining a furthermetallic gasket according to the first embodiment of the presentinvention;

[0100]FIG. 6 is an explanatory sectional view for explaining yet anothermetallic gasket according to the first embodiment of the presentinvention;

[0101]FIG. 7 is an explanatory sectional view for explaining a stillother metallic gasket according to the first embodiment of the presentinvention;

[0102]FIG. 8 is an explanatory sectional view for explaining anadditional metallic gasket according to the first embodiment of thepresent invention;

[0103]FIG. 9 is an explanatory sectional view for explaining a stillfurther metallic gasket according to the first embodiment of the presentinvention;

[0104]FIG. 10 is an explanatory sectional view for explaining a stilladditional metallic gasket according to the first embodiment of thepresent invention;

[0105]FIG. 11 is an explanatory sectional view for explaining an anothermetallic gasket according to the first embodiment of the presentinvention;

[0106]FIG. 12 is an explanatory sectional view for explaining a furthermetallic gasket according to the first embodiment of the presentinvention;

[0107]FIG. 13 is a plan view of an essential part for explaining a stillfurther metallic gasket according to the first embodiment of the presentinvention;

[0108]FIG. 14 is a sectional view taken along the line A-A in FIG. 13;

[0109]FIG. 15 is an explanatory sectional view for explaining anadditional metallic gasket according to the first embodiment of thepresent invention;

[0110]FIG. 16 is an explanatory sectional view for explaining a stilladditional metallic gasket according to the first embodiment of thepresent invention;

[0111]FIG. 17 is an explanatory sectional view for explaining anothermetallic gasket according to the first embodiment of the presentinvention;

[0112]FIG. 18 is an explanatory sectional view for explaining yetanother metallic gasket according to the first embodiment of the presentinvention;

[0113]FIG. 19 is an explanatory sectional view for explaining a furthermetallic gasket according to the first embodiment of the presentinvention;

[0114]FIG. 20 is an explanatory sectional view for explaining a stillfurther metallic gasket according to the first embodiment of the presentinvention;

[0115]FIG. 21 is a plan view for explaining a metallic gasket accordingto a second embodiment of the present invention;

[0116]FIG. 22 is a sectional view taken along the line B-B in FIG. 21;

[0117]FIG. 23 is a sectional view taken along the line A-A in FIG. 21;

[0118]FIG. 24 is a sectional view showing the thickness-increasedportion in another structure according to the second embodiment of thepresent invention;

[0119]FIG. 25 is a sectional view showing the thickness-increasedportion according to yet another structure according to the secondembodiment of the present invention;

[0120]FIG. 26 is a sectional view showing an example of a gasket formedin a multi-layer structure;

[0121]FIG. 27 is a sectional view showing an example of a gasket formedin a multi-layer structure;

[0122]FIG. 28 is a sectional view showing an example of a gasket formedin a multi-layer structure;

[0123]FIG. 29 is a plan view for explaining a metallic gasket accordingto a third embodiment of the present invention;

[0124]FIG. 30 is a sectional view taken along the line A-A in FIG. 29;

[0125]FIG. 31 is a sectional view taken along the line in FIG. 29;

[0126]FIG. 32 is a diagram showing an example in which the bead divergesfrom a full bead into half beads which converge into a full bead;

[0127]FIG. 33 is a sectional view taken along the line D-D in FIG. 32;

[0128]FIG. 34 is a sectional view taken along the line E-E in FIG. 32;

[0129]FIG. 35 is a sectional view taken along the line C-C in FIG. 32;

[0130]FIG. 36 is a reference diagram showing the up-down variation ofthe thickness-increased portion;

[0131]FIG. 37 is a plan view showing a metallic gasket according to afourth embodiment of the present invention;

[0132]FIG. 38 is a sectional view taken along the line A-A in FIG. 37;

[0133]FIG. 39 is a sectional view taken along the line B-B in FIG. 37;

[0134]FIG. 40 is another example of a protrusion formed;

[0135]FIG. 41 is yet another example of a protrusion formed;

[0136]FIG. 42 is a further example of a protrusion formed;

[0137]FIG. 43 is a still further example of a protrusion formed;

[0138]FIG. 44 is a still other example of a protrusion formed;

[0139]FIG. 45 is a yet further example of a protrusion formed;

[0140]FIG. 46 is an additional example of a protrusion formed;

[0141]FIG. 47 is a still additional example of a protrusion formed;

[0142]FIG. 48 is an example in which the base-plate bead is a half bead;

[0143]FIG. 49 is an example in which the base-plate bead is a half bead;

[0144]FIG. 50 is a modification of a second elastic sealing material;

[0145]FIG. 51 is an explanatory sectional view for explaining aconventional metallic gasket;

[0146]FIG. 52 is a sectional view showing a conventional bead;

[0147]FIG. 53 is a plan view showing a conventional converge-divergepart;

[0148]FIG. 54 is a plan view showing a conventional converge-divergepart;

[0149]FIG. 55 is a plan view showing a conventional converge-divergepart;

[0150]FIG. 56 is a plan view showing a conventional converge-divergepart;

[0151]FIG. 57 is a plan view showing a conventional converge-divergepart;

[0152]FIG. 58 is a plan view showing a conventional converge-divergepart; and

[0153]FIG. 59 is a diagram for explaining a fourth conventional metalbead.

BEST MODE FOR CARRYING OUT THE INVENTION

[0154] A first embodiment of the present invention will be describedwith reference to the accompanying drawings.

[0155]FIG. 1 is a plan view of an essential part for explaining ametallic gasket according to an example of a first embodiment of thepresent invention. FIG. 2 is a sectional view taken along the line B-Bin FIGS. 1 and 13. FIG. 3 is a sectional view taken along the line A-A.FIGS. 4 to 20 are diagrams for explaining metallic gaskets in otherforms of the first embodiment of the present invention.

[0156] A metallic gasket as an example of the first embodiment of thepresent invention will be described with reference to FIGS. 1 to 3. Ametallic gasket 10 comprises a base plate 11 made of metal plate, suchas stainless steel plate or soft steel plate. A plurality of combustionchamber openings 12 are formed in the base plate 11 in a manner tocorrespond to the extreme ends of cylinder bores.

[0157] The peripheral end portion on the combustion chamber opening 12side of the base plate 1 is upwardly folded back to form a firstthickness-increased portion X which is the thickest part of the baseplate 11 to give a difference in thickness between thethickness-increased portion and the remaining portions of the base plate11. Similarly, the edge portion on the outer periphery side of the baseplate 11 is upwardly folded back and becomes a secondthickness-increased portion C, and the second thickness-increasedportion C is made thinner by forging than the first thickness-increasedportion.

[0158] As shown in FIGS. 2 and 3, the convex full bead 14 (metal bead)is formed by bending the base plate 11 along each of the seal lines SL1and SL2 on the outer side of the thickness-increased portion X of thebase plate 11. At the same time, a convex full bead (metal bead) whoseheight is equal to or lower than the full bead 14 is formed on the innerside of the second thickness-increased portion C of the base plate bybending the base plate 11. The height of the convex portion 14a of thefull bead 14 is made slightly higher than the first thickness-increasedportion X (by about 0.05 to 0.2 mm depending on the thickness of thefirst thickness-increased portion), and the height of the convex portion15 a of the full bead 15 is made slightly higher than the secondthickness-increased portion C (by about 0.05 to 0.2 mm depending on thethickness of the second thickness-increased portion).

[0159] A plurality of bolt holes 16 for clamping bolts are formed in thebase plate 11 substantially equally spaced in the circumferentialdirection of the combustion chamber hole 12. Water holes 17 are formedbetween the full beads 14 and 15, and an oil hole is formed between thebolt hole 16 and the second thickness-increased portion C. A convex fullbead 19 is formed around the bolt hole 16 in the base plate 11 bybending the base plate 11, and a convex full bead 20 is formed aroundthe oil hole by bending the base plate 11.

[0160] Further, the folded-back portion of the first thickness-increasedportion X of the base plate 11 is made a thin portion 22 in the vicinityof the bolt holes 16, but it is made a thick portion 23 between the boltholes 16 such that the thickness is varied in the circumferentialdirection of the combustion chamber opening 12. The total thickness ofthe thin portion 22 of the first thickness-increased portion X is madethicker than the total thickness of the second thickness-increasedportion C.

[0161] In this embodiment, rubber beads are formed such that an elasticsealing material 24, e.g., a rubber material, fluororubber, NBR orsilicon rubber, a resin material or the like is fixed to the convexportions 14 a, 15 a of the full beads 14, 15 and also to the convexportions (not shown) of the full beads 19, 20, and a similar elasticsealing material 24 is filled in the concave portions 14 b, 15 b on thereverse side of the convex portions 14 a, 15 a of the full beads 14, 15and also to the concave portions (not shown) on the reverse side of theconvex portions (not shown) of the full beads 19, 20.

[0162] On the convex portions of the full beads 14, 15, 19 and 20, anelastic sealing material 24 is fixed in such a manner that its width issubstantially equal to the width of those full beads and the elasticsealing material 24 covers both sides across the width of the width ofthose full beads, and that the height of the elastic sealing material 24is substantially equal to the height of the convex portion of those fullbeads and its top surface is substantially parallel with the flatsurface of the base plate 11.

[0163] On the other hand, the amounts of the elastic sealing material 24filled in the concave portions of the full beads 14, 15, 19 and 20 aresubstantially equal to the capacities of the concave portions and thesurface of the elastic sealing material is substantially flush with theflat surface of the base plate 11.

[0164] In this embodiment, the elastic sealing material 24 is fixed tothe convex portion and filled in the concave portion of the full bead bymolding. For this purpose, passages 21 for molding material are formedin the full beads 14, 15, 19 and 20 to enable simultaneous molding ofthe convex portions and the concave portions of the full beads.

[0165] When a metallic gasket 10 structured as described is sandwichedbetween the joint surfaces of the cylinder block and the cylinder headand fastened with clamping bolts, the elastic sealing material 24 fixedto the convex portion side and filled in the concave portion side of thefull beads 14, 15, 19 and 20 is compressed and deformed in thethrough-thickness direction in cooperation with the deformation of thefull beads, and at the end of fastening, the largest surface pressureconcentrates on the first thickness-increased portion X and the largestload acts on it due to a difference in thickness between the firstthickness-increased portion X with the largest thickness of the baseplate and the remaining portions.

[0166] Therefore, a threefold seal is applied, which comprises thelargest surface pressure on the first thickness-increased portion X, theelastic resilience of the full bead 14 (including the elastic sealingmaterial 24 at the convex portion and the concave portion), and theelastic resilience of the full bead 15 (including the elastic sealingmaterial 24 at the convex portion and the concave portion), andsimultaneously the larger thickness effect of the firstthickness-increased portion X prevents total collapse of the full bead14 and the elastic sealing material 24 fixed to the convex portion sideand filled in the concave portion side of the full bead 14, and thelarger thickness effect of the second thickness-increased portion Cprevents total of the full beads 15, 19 and 20 and the elastic sealingmaterial 24 fixed to the convex portion side and filled in the concaveportion side of the full beads 15, 19 and 20.

[0167] In the metallic gasket 10 structured as described, since theelastic sealing material 24 is fixed to the convex portions 14 a and 15a of the full beads 14 and 15 and the convex portions of (not shown) ofthe full beads 19 and 20 and also the elastic sealing material 24 isfilled in the concave portions 14 b and 1 5 b of the full beads 14 and15 and the concave portions (not shown) of the full beads 19 and 20, theelastic sealing material 24 that protrudes from the base plate 11 is theelastic sealing material only on the convex portion side of the fullbeads, in other words, on one side of the base plate 11.

[0168] Therefore, the height of the elastic sealing material 24 fixed tothe convex portion 14 a of the full bead 14, for example, is t0+thecompression-deformed amount (t0

0.4(40% max.)) if the thickness of the folded-back portion of the baseplate 11 at the first thickness-increased portion X is designated as t0by referring to FIG. 2, and assuming that t0 is 0.5 mm, the height ofthe rubber bead is 0.7 mm from the above equation, which is twice the0.35 mm in prior art shown in FIG. 51.

[0169] Therefore, the elastic sealing material 24 on the convex portion14 a side of the full bead 14 increases in quantity, and theircompression-deformed amounts become larger, making it easy to carry outa forming process of the elastic sealing material, and making itpossible to increase the thickness of the elastic sealing material partso that it becomes possible to set a larger processing tolerance andreduce manufacturing cost.

[0170] Further, the elastic sealing material 24 fixed to both sidesacross the width of the convex portions 14 a and 15 a of the full beads14, 15 is exposed to cooling water only at the areas facing the waterhole 17 side. On the other hand, the elastic sealing material 24 filledin the concave portions 14 b, 15 b of the full beads 14, 15 is coveredwith the full beads 14, 15 and is not exposed to cooling water, forwhich reason the elastic sealing material is protected fromdeterioration and therefore can maintain a stable seal performance for along period of time.

[0171] Further, a required sealing pressure is obtained by synergy ofthe resilience of the full beads 14, 15, 19 and 20 and the elasticresilience of the elastic sealing material 24 fixed to the convexportion side and filled in the concave portion side of the full beads14, 15, 19 and 20, a fact which makes it possible to decrease thehardness of the base plate, therefore eliminate worries about fatiguefailure of the beads of the base plate, absorb the engine vibrationamplitude and the seal area roughness, and thereby seal cooling waterpressure and oil pressure with a lower surface pressure.

[0172] Further, since a wide sealing area can be secured for the elasticsealing material 24 on the convex portion side of the full beads 14, 15,19 and 20 and for the elastic sealing material 24 on the concave portionside of the full beads 14, 15, 19 and 20, the flaws on the joint surfaceand the blowholes that occur in casting can be sealed satisfactorilywith a low surface pressure. Furthermore, since the elastic sealingmaterial 24 consist of an elastic substance (rubber-based material,above all else), the gasket factor is low, and therefore a limited axialload can be utilized effectively in the areas under adverse condition,so that total load can be decreased.

[0173] Further, the first thickness-increased portion X of the baseplate 11 is made a thin web area 22 in the vicinity of the bolt holes16, but it is made a thick web area 23 between the bolt holes 16 suchthat the resilience of the first thickness-increased portion X duringfastening the bolts is weak in the vicinity of clamping bolts whosefastening force is large, but the resilience is stronger in the areabetween the clamping bolts whose fastening force is relatively small.Consequently, the surface pressure applied to the firstthickness-increased portion X can be equalized in the circumferentialdirection of the combustion chamber opening 12 and the axial tension ofthe clamping bolts can be decreased, thus making it possible toeffectively prevent deformation of the engine, especially, in engines oflow rigidity.

[0174] The structure of the metallic gasket according to this inventionis not intended as a definition of the limits of the invention, but maybe changed at discretion without departing from the scope and the spiritof the invention.

[0175] For example, in the embodiment mentioned above, the secondthickness-increased portion C is made thinner by forging after foldingback the edge portion on the outer periphery side of the base plate 11.As an alternative for this, as shown in FIGS. 4 and 7, for example, thesecond thickness-increased portion C may be formed by attaching, bywelding for example, a shim plate 30 thinner than the base plate may bemounted to the outer peripheral region of the base plate 11 to therebyomit the above-mentioned forging of the second thickness-increasedportion C. As shown in FIG. 6, a shim plate formed as a full bead 31 maybe included in the folded portion of the thickness-increased portion X,or as shown in FIG. 9, a flat shim plate 33 may be included in thefolded portion of the first thickness-increased portion X to therebyomit the forging of the second thickness-increased portion C.

[0176] Further, in the above-mentioned embodiment, the firstthickness-increased portion X is formed by folding back the end portionon the combustion chamber opening 12 side of the base plate 11, but asan alternative for this, as shown in FIG. 7, the firstthickness-increased portion X may be formed by attaching a shim plate 34substantially equal in thickness to the base plate 11 to the end portionon the combustion chamber opening 12 side of the base plate 11 bywelding, for example. Or, the first thickness-increased portion X may beformed by fitting a grommet 35 over the end portion on the combustionchamber opening 12 side of the base plate 11 as shown in FIG. 10, or byfitting a grommet 37 over the end portion on the combustion chamberopening 12 side of the base plate 11 through the intermediary of anelastic plate 36 as shown in FIG. 11, or by attaching a shim plate 32formed as a full bead 31 to the end portion on the combustion chamberopening 12 side of the base plate 11 by welding, for example, as shownin FIG. 17.

[0177] Further, in the above-mentioned embodiment, description has beenmade of an example that the full bead 14 is formed on the outer side ofthe first thickness-increased portion X of the base plate 11 and a fullbead 15 is formed on the inner side of the second thickness-increasedportion C of the base plate 11, and then the elastic sealing material 24is fixed to the convex portions 14 a and 15 a and is filled in theconcave portions 14 b and 15 b of the full beads 14 and 15. However,this is not intended as a definition of the limits of the invention, andas shown in FIGS. 5 and 8, on the inner side of the secondthickness-increased portion C of the base plate 11, in place of the fullbead 15, a half bead 45 in a stepped structure may be formed by bendingthe base plate 11, and an elastic sealing material 24 may be fixed tothe convex portion 45 a and filled in the concave portion 45 b on thereverse side of the convex portion of the half bead. Further, as shownin FIG. 8, in place of the full bead 14, on the outer side of the firstthickness-increased portion X of the base plate 11, a half bead 44 in astepped structure may be formed by bending the base plate 11, and anelastic sealing material 24 may be fixed to the convex portion 44 a andfilled in the concave portion 44 b of the half bead 44.

[0178] The elastic sealing material 24 filled in the concave portions 44b and 45 b of the half beads 44 and 45 is so formed as to besubstantially flush with the flat surface of the base plate 11, andtherefore the elastic sealing material is not exposed to cooling water,but the elastic sealing material 24 fixed to the convex portions 44 aand 45 a protrudes from the flat surface of the base plate and isdirectly exposed to cooling water. In this case, by providing theelastic sealing material 24 fixed to the convex portions 44 a and 45 awith a wide width, the elastic sealing material 24 in its entirety canbe prevented from deteriorating, thereby ensuring its superior sealingperformance.

[0179] Further, as shown in FIGS. 8 and 9, a third thickness-increasedportion B may be formed by providing a thick plate 50 substantiallyequal in height to or thinner than the base plate 11 between the halfbeads 44 and 45 or between the full beads 14 and 15 of the base plate11, thereby preventing the engine from being deformed by receiving loadon a wide area in engines with large total load, such as a dieselengine. In this case, the relation among the thickness T1 of the firstthickness-increased portion X, the thickness T2 of the thirdthickness-increased portion B, and the thickness T3 of the secondthickness-increased portion C is set as T1>T2 T3.

[0180] Further, in the embodiment mentioned above, the height of theconvex portion 14 a of the full bead 14 is made a little higher than thefirst thickness-increased portion X and substantially equal to theheight of the elastic sealing material 24 fixed to the convex portion 14a, and the height of the convex portion 15 a of the full bead 15 is madea little higher than the second thickness-increased portion C. However,these are not necessarily required, and for example, as shown in FIG.12, the height of the convex portion 14 a of the full bead 14 may bemade a little lower than the first thickness-increased portion X so thatthe full bead 14 is embedded in the elastic sealing material 24, andthough this is not shown graphically, the convex portion of the fullbead may protrude from the elastic sealing material 24. The same appliesto the half beads, too.

[0181] Further, in the embodiment mentioned above, the surface of theelastic sealing material 24 fixed to the convex portion side of the fullbead or the half bead is made substantially parallel with the flatsurface of the base plate 11, but this is not necessarily required, andas long as the elastic sealing material 24 can be compressed anddeformed in cooperation with the bead, various shapes may be adopted. Inother words, the compressed amount is set so as not to cause compressionfracture, various shapes may be adopted for the sealing material.

[0182] Further, in the above-mentioned embodiment, the elastic sealingmaterial 24 filled in the concave portion of the half bead or the fullbead is made substantially flush with the flat surface of the base plate11, but as long as the elastic sealing material 24 is compressed anddeformed in the through-thickness direction in cooperation with thebead, the elastic sealing material 24 may have a slightly rough surfacewith respect to the flat surface of the base plate 24. For example, asshown in FIG. 18, the elastic sealing material 24 filled in the concaveportion of the full bead 14 may have a groove 24 a formed therein topermit the elastic sealing material 24 to deform easily.

[0183] Further, in the above-mentioned embodiment, description has beenmade of a case where the elastic sealing material 24 fixed to the convexportion side of the full bead is disposed at both sides in the widthdirection of the convex portion, but this arrangement is not intended asa definition of the limits of this invention, and as shown in FIGS. 15and 16, the elastic sealing material 24 fixed to the convex portion sideof the full bead 14 may be placed only on the combustion chamber opening12 side of the convex portion 14. By this arrangement, the elasticsealing material 24 (rubber, for example) on the convex portion side isprevented from being exposed to cooling water or oil to thereby improvedurability, and this structure is most suitable when the full bead 14 isclose to a bolt hole 16 or a water hole 17. The width of the bead may bevaried in the circumferential direction to adjust the surface pressure.

[0184] Further, though not shown in FIGS. 4 to 12, 15 to 18, needless tosay, it is possible to bore molding-material passages 21 in the fullbeads 14, 15, 19 and 20 and the half beads 44 and 45 in a metallicgasket shown in FIGS. 15 and 18, 1 qand carry out simultaneous formingof a sealing material on the convex portion side and on the concaveportion side.

[0185] In each of the above-mentioned embodiments, the elastic sealingmaterial 24 has been filled in the concave portion of the bead so as tobe substantially flush with the flat surface of the base plate 11 beforethe cylinder block and so on are fastened together by clamping bolts.Alternatively, it is possible to fill the concave portion of the fullbead with an elastic sealing material in an amount substantially equalto the capacity of the concave portion, and arrange for the elasticsealing material 24 in the concave portion to be compressed and deformedso as to be substantially flush with the flat surface of the base plate11 when the engine is assembled by fastening by clamping bolts.

[0186] Further, not shown in FIGS. 2-8, 10-12, and 15-18, a lubricant40, such as molybdenum disulfide, which provides sealing and slidingproperties, may be applied to one or both surfaces of the base plate 11to prevent fretting caused by displacement or knocking by a thermalexpansion difference between the engine and the gasket, or vibrationamplitude as shown in FIG. 9.

[0187] Further, in the above-mentioned embodiment, description has beenmade referring to a case, for example, where the full bead 14 isprovided on the radially outer side of the first thickness-increasedportion X at each combustion chamber opening 12, but this invention isnot limited to this structure, and as shown in FIGS. 13 and 14, it goeswithout saying that this invention can be applied to a type of metallicgasket in which the full bead 14 cannot be placed between the combustionchamber openings 12 because the distance between cylinders (combustionchamber openings) has come to be narrow by weight-saving and down-sizingby the trend toward all-aluminum and high-powered automobiles.

[0188] As in the example shown in FIG. 15 or the like, to use theelastic sealing material 24 only on the thickness-increased portion Xside is disadvantageous from a standpoint of inhibiting deformation ofthe base plate by means of the spring force of the elastic sealingmaterial on the concave portion side. From a standpoint of inhibiting adecrease in sealing pressure by restraining the deformation, when anelastic sealing material is placed only on one surface side, it isdesirable to locate the elastic sealing material 24 on the side oppositethe thickness-increased portion X or C as shown in FIG. 19. This isbecause the thickness-increased portion X or C inhibits the base platefrom being deformed on the thickness-increased portion X or C side. Thisapplies in the vicinity of the portions fastened by bolts.

[0189] Further, in the above-mentioned embodiment, the elastic sealingmaterial 24 is applied to the whole surface or the one-side surface ofthe convex portion of the metal bead 15, which, however, is not intendedas a definition of the limits of the invention. As shown in FIG. 20, theelastic sealing material 24 may be applied to the flat portion of thebase plate which is continuous to the metal bead 15, which, however,makes the bead width wider and the elastic sealing material is morelikely to separate because of its not being fixed to the surface of theconvex portion.

[0190] Next, a second embodiment of the present invention will bedescribed with reference to the accompanying drawings.

[0191]FIG. 21 is a plan view for explaining a metallic gasket 1according to the second embodiment of the invention. FIG. 22 is asectional view taken along the line B-B, in other words, at an endposition of the boundary area X in FIG. 21. FIG. 23 is a sectional viewtaken along the line A-A, in other words, at the center position of theboundary area X in FIG. 21.

[0192] Description will start with the structure of the metallic gasketaccording to this embodiment.

[0193] The metallic gasket 1 according to this embodiment is an exampleof a metallic gasket 1 sandwiched between opposing joint surfaces of thecylinder head and the cylinder block of an internal combustion engine ofa multicylinder structure.

[0194] As a material for the base plate 2 of the metallic gasket, it ispossible to adopt as necessity requires metal plate, such as stainlesssteel, soft steel or aluminum. Here, in order to provide a lessexpensive metallic gasket 1, suppose that soft steel plate of lowrigidity is used.

[0195] As shown in FIG. 21, in the center area of the base plate 2, aplurality of large openings for combustion chamber bores 3 are arrangedin the longitudinal direction. Further, a plurality of bolt holes 4 areformed in the area along the outer circumference of the base plate 2.

[0196] The region of the internal circumference encircling the openingend of each combustion chamber opening 3 is folded back upwardly to forma thickness-increased portion 6 of largest thickness, by which athickness difference is provided with respect to the remaining portions.In a boundary area X between the adjacent combustion chamber openings,because the clearance between the combustion chamber openings 3 isminimum at the center position, the width of the thickness-increasedportion 6A on each side of the boundary at the center position is set asan extremely small value and the width of the thickness-increasedportion 6 is made to become wider toward each end of the boundary areaX, and at each end of the boundary area X, the width of thethickness-increased portion 6B on each side of the boundary is madewider than in other parts of the area X to make up for the narrowthickness-increased portion in the center position of the boundary areaX.

[0197] Further, on the ground that each end side of the boundary area Xis close to the bolt hole 4, the height of the thickness-increasedportion 6 at each end of the boundary area X is made slightly lower thanin the center. Position (the end portion is lower by in FIG. 22) tothereby equalize the surface pressure applied to the thickness-increasedportion 6 in the circumferential direction (in the extending direction)around the combustion chamber opening. In other words, looking at thethickness-increased portion 6 at the boundary area X, for example, onecan see the height at 6A on each side of the boundary at the centerposition is made relatively higher than the height of the end portion at6B so that a required sealing pressure can be generated in thethickness-increased portion 6A of narrow width.

[0198] Here, in this embodiment, because soft steel can be adopted asmaterial for the base plate 2, it is easy to fold back the base plate toform the thickness-increased portion 6 and change thethickness-increased portion 6 easily.

[0199] Further, on the outer side of the vicinity of thethickness-increased portion 6, a seal line Y of the bead of an endlesscircle is set to encircle each combustion chamber opening 3, and aconvex full bead 5 is formed along the seal line Y by bending the baseplate 2.

[0200] The seal lines Y converge into one seal line YA shared by theadjacent combustion chamber openings 3 when the seal lines run into theboundary area X between the adjacent combustion chamber openings 3.

[0201] Elastic sealing material 7A, 7B are fixed to the surface of thebase plate 2 on the convex portion side of the full bead 5, and asimilar elastic sealing material 8 is filled in the concave portion onthe reverse side of the convex portion side. The elastic sealingmaterial 7A, 7B and 8 are made of a heat-resistant and elastic material,such as a rubber material of fluororubber, NBR or silicon rubber, aresin material or the like.

[0202] The elastic sealing material 7A and 7B fixed to the specifiedsides of the convex portion of the full bead 5 are arranged to coverboth sides of the surface of the convex portion and also slightly coverthe flat area continuous to the convex portion. In this embodiment, thetotal width of the elastic sealing material 7A and 7B on the convexportion side is set to be a little wider than the width of the bead.That width may be about equal to the bead width. The height of theelastic sealing material 7A, 7B is made substantially equal to theheight of the bead.

[0203] Elastic sealing material 7A, 7B which has a height equal to theheight of the bead 5 may be applied to the flat area of the base platecontinuous to the convex portion of the bead 5 as described above, butit is desirable to set the width of the rubber bead consisting of theelastic sealing material 7A, 7B so as to be not more than about 1.5times the width of the bead 5. The height of the bead 4 and the elasticsealing material 7A, 7B is set so that the compression rate of theelastic sealing material 7A, 7B and 8 on both surfaces is within 35%when compressed to the height of the thickness-increased portion tothereby prevent compression fracture of the elastic sealing material 7A,7B and 8.

[0204] Suppose that a high-pressure gas from the combustion chamber bore3 side leaks through the minute channels formed as tool marks on thejoint surfaces facing the thickness-increased portion 6. Even if thisoccurs, the rubber bead consisting of the elastic sealing material 7A,7B and 8 can completely seal the leak of high-pressure gas because thosechannels are filled by the elastic sealing material 7A, 7B when thosesealing material underwent elastic deformation along the seal line Y ofthe bead 5.

[0205] Even if cooling water intrudes from the outer side (outerperiphery side) of the rubber bead, the flexible rubber bead can sealoff the water by completely closing even the tool marks. The elasticsealing material 8 filled in the concave portion is not directly exposedto cooling water and does not deteriorate.

[0206] On the other hand, the elastic sealing material 7A, 7B on theconvex portion side contacts at their end faces which are close to thecooling water hole. Even when the portion of the sealing materialdeteriorates by contact with the cooling water from using anundesignated cooling water and the performance of the elastic sealingmaterial 7A, 7B, that part of the elastic sealing material 7A, 7B on theinner side of the bead 5 is separated by the convex portion of the bead5 and does not directly contact the cooling water. All in all, acomplete seal can be guaranteed for a long period of time. Note that theaction of the cooling water mentioned above is its action along the sealline other than at the boundary area X.

[0207] Further, with regard to the rubber bead width, assuming that thewidth of the narrow elastic sealing material 8 on the concave portionside is about 2.5 mm, since the width of the convex portion side can bedesigned to be wider than that, as long as the casting-blowholes are notwider than the bead width, they can be sealed. Even if flaws made intransit has been formed in a manner to run across the bead width, acomplete seal can be obtained by closing up the flaws on the sealsurface by area seal using the rubber bead.

[0208] Further, in this embodiment, since the seal seals the joint by acomposite spring of the bead 5 made of soft steel and the rubber beadmade of soft elastic sealing material 7A, 7B, an extremely high surfacepressure cannot be expected, but the surface pressure can be adjusted tosome extent by increasing the width of the rubber bead or by increasingthe hardness of rubber. Since both contact surfaces are formed by softelastic sealing material 7A, 7B or 8, and the gasket factor can bedecreased, with the result that large load is not required, the totalfastening load, in other words, the fastening axial tension of bolts canbe reduced.

[0209] Further, as rust-preventive measures of the base plate 2, it ispossible to adopt plating finish for the end faces of the base plate 2and the sectional areas of the holes or apply a ultra-thin rubbercoating or molding coating to all parts that may contact cooling waterand the parts located on the outer side of the bead 5, which may suffercorrosion.

[0210] As has been described, in this embodiment, even if the clearancebetween adjacent combustion chamber openings 3 becomes increasinglysmaller by reductions in size and weight of engines or if blowholesoccur on the joint surfaces by progressive use of aluminum, it ispossible to securely apply an effective seal by the bead 5 and byelastic sealing material 7A, 7B or 8, and it is also possible to reducethe fastening axial tension. Since it has become possible to adopt softsteel of low rigidity for the base plate 2, the workability of themetallic gasket 1 can be improved.

[0211] More specifically, the bead 5 is formed on the base plate 2 ofsoft steel and the rubber bead consisting of elastic sealing material7A, 7B and 8, which have elasticity, is formed on both surfaces of thebead 5, and spring forces are generated by the compression-deformedamounts of the bead 5 of the base plate 5 and the rubber bead. Thecompression-deformed amounts of the base-plate bead 5 and the rubberbead are limited by the thickness of the thickness-increased portion 6formed by folding back the peripheral end portion on the combustionchamber opening 3 side of the base plate. Owing to the use of the baseplate 2 made of soft steel material of low rigidity, the hardness bywork hardening does not rise much, and though spring forces aregenerated by the rubber bead applied to both surfaces of the bead 5,because the rubber bead is made of soft rubber, deformation load issmall, and most of the load is designed to be received by thethickness-increased portion. Consequently, while the engine isoperating, vibration amplitude is smaller than before, and there is noworry about fatigue failure. The height of the thickness-increasedportion is varied partly along the internal circumference of thecombustion chamber opening 3, and for example, that height is low in thevicinity of bolts, and is made high between the bolts and between thebores to thereby average the fastening surface pressure in the regionencircling the combustion chamber opening 3. Accordingly, a highpressure can be sealed by the width of the thickness-increased portionof the minimum width in engines which have a narrow clearance betweenthe combustion chamber openings 3. High-pressure gas is sealed by thethickness-increased portion while low-pressure cooling water and oilpressure are sealed by the rubber bead line. The rubber bead line iscapable of adapting itself to the surfaces of the engine, the gasketfactor is small, so that total load can be reduced.

[0212] The rubber bead line, having a wide width, can sealcasting-blowholes of sizes falling within the width. The flaws that runthrough the range of the bead 5 can be sealed by the elastic sealingmaterial 7A, 7B and 8 that come into close contact with the flaws whenthey are bent. Further, since the base plate 2 is soft steel and lessexpensive than stainless steel, it is possible to provide ahigh-performance gasket at low price.

[0213] As shown in FIGS. 22 and 23, when the thickness-increased portion6 is formed by bending the peripheral end portion on the combustionchamber opening 3 side of the base plate 2, the width of thethickness-increased portion becomes smaller by an amount taken up toform the radius R of the folded portion (the portion facing thecombustion chamber bore 3). In consideration of this, to increase thewidth of the thickness-increased portion to thereby increase the sealarea by the thickness-increased portion, instead of forming thethickness-increased portion by bending the end portion, it is onlynecessary to fix a separate subplate 10 to the base plate 2 as shown inFIG. 24. In this case, the width of the thickness-increased portion canbe increased by an amount which would otherwise be taken up to form therounded portion.

[0214] Further, the thickness-increased portion may be formed as shownin FIG. 25, in which the subplate 11 is bent at each end of it toincrease the thickness.

[0215] When the metallic gasket 1 requires a larger thickness, aplurality of plates are stacked as shown in FIGS. 26 to 28.

[0216] The structure of the gasket in FIG. 26 is formed as follows. Intwo metallic gasket 1, each having formed therein a full bead 5, towhich elastic sealing material 7A, 7B and 8 are attached, as describedabove, the full beads 5 are arranged with convex portions facing eachother, the flat surfaces on the concave portion side of the metal beads5 are brought into contact with the joint surfaces, and the subplate 12,which has a folded thickness-increased portion formed on each side, issandwiched between the two metallic gaskets 1. In the subplate 12, thewhole areas of the end portions contacting the elastic sealing material7A, 7B are made to protrude at the folded end portions at opposite ends,and the deformed amount of each of the beads 5 of the couple of metallicgaskets 1 is thereby adjusted to not more than 35%, for example.

[0217] The subplate 12 may be formed by soft steel, and by partlyvarying the thickness of the folded, thickness-increased portion, thesurface pressure is equalized after fastening of bolts. The requiredthickness of the whole gasket is adjusted according to the thickness ofthe subplate 12.

[0218] In the structure under discussion, a high pressure gas from thecombustion chamber bore 3 or cooling water that has entered from theouter periphery side infiltrates between the pair of metallic gaskets(more precisely, between each metallic gasket 1 and the subplate 12).Therefore, this gasket is not affected by the blowholes or tool marks onthe joint surfaces of the cylinder head or the like, and can exhibitsthe same effects as described above. In this case, it is possible tosecure an enough thickness for the elastic sealing material 7A and 7B,thus increasing the gasket adaptability to deformation. Those effectsare obtainable also with the examples in FIGS. 27 and 28.

[0219] The structure of the gasket in FIG. 27 is such that a subplate 13having a folded, thickness-increased portion is disposed to face theconvex portion side of the bead 5 of the metallic gasket 1, the metallicgasket 1 has a full bead 5 holding the elastic sealing material 7A and7B, mentioned above.

[0220] The structure of the gasket in FIG. 28, which resembles thestructure in FIG. 26, has two subplates 14 and 15 sandwiched between thepair of metallic gaskets 1. Out of the two subplates 14 and 15, onesubplate 14 has a step, that is, a difference in surface level formed ateach end portion thereof, and the other subplate 15 has each end portionthereof folded back, and each of the folded-back end portion is locatedwhere there is the lower-level portion. An upper half of the thicknessof the folded-back end portion of the subplate 15 serves as a stopperagainst the upper bead and a lower half of it serves as a stopperagainst the lower bead. (The deformed amount of the lower rubber bead isrestrained by elimination of the gap between the two subplates 14 and15.)

[0221] Note that the above-mentioned examples are not intended as adefinition of the limits of the invention but this embodiment is capableof developing limitlessly by combinations of this technology of thethickness-increased portion and new rubber beads.

[0222] With regard to the structure, operation and effects of the secondembodiment, those mentioned in the first embodiment apply.

[0223] Next, a third embodiment of the present invention will bedescribed with reference to the accompanying drawings.

[0224]FIG. 29 is a plan view for explaining a metallic gasket in thethird embodiment. FIG. 30 is a sectional view taken along the line A-Ain FIG. 29. FIG. 31 is a sectional view taken along the line B-B in FIG.29.

[0225] Description will start with the structure of a metallic gasketaccording to the third embodiment.

[0226] The metallic gasket 1 is an example of a cylinder head gasket.The base plate 2 of the metallic gasket 1 is made of thin metal plate,such as stainless steel, soft steel and aluminum with low hardness.

[0227] In the center of the base plate 2 there are formed combustionchamber openings 3 aligned to each other in the longitudinal direction,and a first thickness-increased portion 6 is formed by upwardly foldingback the end portion of the internal circumference of the combustionchamber opening 3. This first thickness-increased portion 6 is used toseal off a high-pressure gas of the combustion chamber.

[0228] A first seal line SL1 is provided on the outer side of the firstthickness-increased portion 6 in a manner to endlessly encircle eachcombustion chamber opening 3. Because the first seal lines SL1, eachencircling the adjacent combustion chamber opening 3, have to becomenarrow in the boundary area between the adjacent two combustion chamberopenings 3, the first seal lines SL1 merge or split at the inlet oroutlet of the boundary area, and therefore the seal lines SL1 share oneseal line in the boundary area. The part where two seal lines cometogether or split forms a converge-diverge area M.

[0229] A plurality of cooling water holes 4 are formed in the base plate2 on the outer side of the first seal line SL1, and a second seal lineSL2 is provided in a manner to endlessly encircle the whole a pluralityof cooling water holes 4 and combustion chamber opening 3. Bolt holes 9and an oil hole 5 are formed on the outer side of the second seal lineSL2, and third seal lines are formed in a manner to endlessly encirclethe bolt holes 9 and the oil hole 5. The second and third seal lines SL2and SL3, where they come together, are made to merge into one seal line.In other words, the part where the seal lines come together or splitforms a converge-diverge part M.

[0230] In part of the external end portion of the base plate 2, secondthickness-increased portions 12 are formed by upwardly folding back theend portion. The height of the second thickness-increased portion 12 isset to be a little lower than the minimum height in the firstthickness-increased portion 6 by forging.

[0231] The number of the converge-diverge parts M of the seal linesincreases when the seal lines are designed to be short in length or anenough width of seal line is hard to obtain for design reasons.

[0232] The bead width of the metal bead 7 is designed to be constanttoward the converge-diverge part M insofar as possible as shown in FIG.43. Needless to say, this example is not intended as a definition of thelimits of the invention.

[0233] The bead that forms the seal line is a composite body of a metalbead 7 and a rubber bead 8 as shown in FIGS. 30 and 31.

[0234] The metal bead 7 according to the third embodiment is a full beadformed by bending the base plate 2 so that the bead protrudes in aconvex form only on one surface side (upper side) of the base plate 2.The height of the metal bead is set to be higher than the firstthickness-increased portion 6, so that the bead undergoes elasticdeformation in the through-thickness direction, thereby generating asealing pressure.

[0235] The rubber bead 8 comprises an elastic sealing material 8 b fixedto the surface of the base plate 2 on the convex portion side of themetal bead 7, and an elastic sealing material 8 a filled in the concaveportion located on the reverse side of each convex portion of the metalbead 7. The elastic sealing material 8 a and 8 b may be formed by arubber material, such as fluororubber, NBR and silicon rubber, a resinmaterial, or the like, which has heat resistance, corrosion resistance,weatherability and elasticity.

[0236] The elastic sealing material 8 b fixed to the convex portion sideof each metal bead 7 is fixed on the surface of the base plate 2 in amanner to cover at least both sides of the width of the metal bead 7.The height of the elastic sealing material 8 b fixed to the convexportion side of each metal bead 7 is set to be equal to or substantiallyequal to the height of the convex portion of each metal bead 7, and thetop surface is set to be substantially parallel to the flat surface ofthe base plate 2. The height of the elastic sealing material 8 b may beset to be slightly higher than the height of the convex portion of themetal bead 7. However, the height of the elastic sealing material 8 bshould be a height at which the compression-deformed amount regulated bythe height of the first thickness-increased portion 6 can be restrainedto not more than 35% (the compression-deformed amount without subject tobuckling, which is already known for the material).

[0237] On the other hand, the amount of the elastic sealing material 8 afilled in the concave portion of each metal bead 7 is set to besubstantially equal to the capacity of the concave portion andsubstantially flush with the flat surface of the base plate 2.

[0238] The width of the metal bead 7 along the bead line issubstantially constant, and generally the width of the metal bead 7becomes relatively wider at the converge-diverge part M than in theremaining portions of the bead. (Refer to FIGS. 30 and 31) For thisreason, as long as the metal bead 7 is concerned, the spring force atthe converge-diverge part M, but as shown in FIG. 31, the amount of theelastic sealing material 8 a and 8 b filled in the concave portion isconsiderably large, so that the deformation load at the converge-divergepart M becomes large.

[0239] In view of this, with regard to the rubber bead 8 on the convexportion side of the metal bead 7, the elastic sealing material 8 a and 8b are applied to the convex portion slope of the metal bead 7 and alsoto the flat surface of the base plate 2 in the remaining portions of thebead, but in the converge-diverge part M, the elastic sealing material 8a and 8 b are applied only to the convex portion slope of the metal bead7 to thereby decrease the deformation load. Note that for the absolutewidth of the rubber bead 8, as shown in FIG. 31, it is wider at theconverge-diverge part M to compensate for a decrease in the spring forceof the metal bead 7 which is made wide.

[0240] As has been described, the ratio of the width of the rubber bead8 to the width of the metal bead 7 is made different between in theconverge-diverge part M and in the remaining portions of the bead tothereby equalize the sealing pressures along the seal line and at theconverge-diverge parts M. In the above example, the ratio of the widthof the rubber bead 8 to the width of the metal bead 7 is set so as to besmaller at the converge-diverge parts M.

[0241] As stated above, only by adjusting the width of the rubber bead 8capable of easy width adjustment, the sealing pressure along the seallines can be equalized regardless of the presence of manyconverge-diverge parts or without any need to worry about the figurationof the converge-diverge part of the metal bead 7 along the bead line.

[0242] The rubber bead 8 having sealing material formed on both surfacesof the metal bead 7 has by itself a lower spring force than the bead ofhigh-hardness base plate though this depends on the hardness of thesealing material. However, when the rubber bead 8 and the metal bead 7are combined as a composite bead, the composite bead provides an areaseal by the elastic sealing material 8 a and 8 b, which have a betteradaptability to the joint surfaces of the engine, so that the gasketfactor can be restricted to a small value. In other words, if thehardness of the base plate 2 is decreased and the spring force of themetal bead 7 becomes smaller, there is no problem, and accordingly thereis no need to worry about cracks.

[0243] As described above, when the gasket factor becomes smaller,because the gasket factor of 2 to 4 is sufficient with the margin ofsafety ratio taken into consideration, the total fastening load can bedecreased.

[0244] An area seal formed by the elastic sealing material 8 a and 8 bis applied, which offers significant sealing effects covering theprocessing roughness of the joint surfaces, flaws running across thebead width, and blowholes that occur during aluminum casting.

[0245] For the metal bead 7 of the third seal line SL3 along the outerperiphery of the base plate 2, where the seal width is narrow, a metalbead 10 in the form of a half bead of a stepped structure is sometimesadopted. More specifically, after the full bead in FIG. 33 becomes widerin the converge-diverge part as shown in FIG. 34, the bead splits intotwo half beads 10 of a stepped structure as shown in FIG. 35, and thetwo half beads merge into one full bead.

[0246] When a full bead splits into two half beads 10 which in turnconverge as mentioned above, in view of the fact that at the time ofcompression, the elastic sealing material bends sideways and the springforce weakens, the width of the rubber portion is made relatively wideas shown in FIGS. 33 and 34 to thereby equalize the pressure.

[0247] At this time, it is desirable to widen the width of the rubberbead in the vicinity of the bolts.

[0248] In this embodiment, the thickness-increased portions is made toreceive a greater part of the bolt-fastening load and a large pressureis applied to the first thickness-increased portion 6 encircling thecombustion chamber opening 3 to thereby seal a high-pressure combustiongas by the first thickness-increased portion. Under this condition, bylimiting the deformed amount of the bead to a proper value, it becomespossible to prevent compression fracture of the elastic sealing material8 a and 8 b. It ought to be noted that for the metallic gasket 1 usedfor the parts where the fastening axial tension is low, the first andsecond thickness-increased portions 3 and 12 are not required.

[0249] The deformation of the combustion chamber bore is caused bydifferences in surface pressure between in the vicinity of bolts and inthe region between the bolts depending on the rigidity of the enginewhen the cylinder block and the cylinder head are fastened together witha metallic gasket 1 is placed between the joint surfaces thereof

[0250] By partly varying the thickness of the region of thecircumference of the first thickness-increased portion 6 facing acombustion chamber bore (Refer to FIG. 36) so as to equalize the sealingpressure along the extending direction of the above-mentioned portion 6,and thus the bore is prevented from being deformed to thereby maintainthe roundness of the bore. The more the roundness of the bore isimpaired, the larger the oil consumption and power loss will be.

[0251] A fourth embodiment of the present invention will be describedwith reference to the accompanying drawings.

[0252]FIG. 37 is a plan view for explaining a metallic gasket 1according to the fourth embodiment.

[0253] The metallic gasket 1 according to the present inventioncomprises a base plate 2 made of a metal plate, such as stainless steelor soft steel. In the basic concept of this embodiment, a sheet of baseplate 2 is used. However, more than one base plate 2 may be used whenoccasion requires.

[0254] In the following, a case will be described where the metallicgasket 1 is formed by a sheet of base plate 2.

[0255] Description will be made of the structure, in which a combustionchamber opening 3 is formed almost in the center of the base plate 2.Bolt holes 4 and an oil hole 5 are formed on the radially outer side ofthe combustion chamber opening 3. A chain chamber hole 17 is alsoformed.

[0256] As shown in FIG. 38, the peripheral end portion on the combustionchamber opening 3 side of the base plate is upwardly folded back to forma thickness-increased portion 16 which is thicker than the remainingportions of the base plate. In this embodiment, the thickness-increasedportion 16 is formed by folding back, but it may be formed by any ofwell-known methods, such as fixing a flat plate.

[0257] Seal lines SL1 and SL2 are arranged in a manner to encircle eachof the holes, and along each of the seal lines SL1 and SL2 a, a bead isformed.

[0258] As shown in FIGS. 38 and 39, a bead BD in this embodiment isformed by combining a base plate 6 as a full bead and rubber beads 8 and10.

[0259] The base-plate bead 6 is formed by bending the base plate 2 inthe through-thickness direction, and is in a convex form on thethickness-increased portion side 16 so as to be higher than thethickness-increased portion 16.

[0260] The above-mentioned rubber bead is formed by a first elasticsealing material 10 filled in the concave portion of the base-plate bead6, and a second elastic sealing material 8 fixed to the convex portionside of the base-plate bead 6.

[0261] The first elastic sealing material 10 is set in such a way thatits flat surface is substantially flush with the underside of the baseplate 2, and in substantially the center position of the bead width, aprotrusion extending downwardly is formed along the seal lines SL1 andSL2.

[0262] The second elastic sealing material 8 is formed on the surface ofthe bead convex portion, extending slightly to a flat surface continuousto the convex portion. The height of the second elastic sealing material8 is designed to be at substantially the same height as the base-platebead 6 and to have a substantially flat surface (top surface).

[0263] Preferably, the width of the second elastic sealing material 8 isnot more than 1.5 times the width of the base plate 6. If the width istoo wide, load will be needlessly increased. The height of the secondelastic sealing material 8 is preferably in a range of 0.9 to 1.1 theheight of the base-plate bead 6.

[0264] The number of and the kinds of holes, such as bolt holes 4, andthe positions of the seal lines SL1 and SL2 naturally differ with thekinds of the cylinder head and the cylinder block between which themetallic gasket 1 is disposed.

[0265] The metallic gasket 1 structured as described is mounted when itis sandwiched between the joint surfaces of the cylinder block and thecylinder head of an engine, the beads are deformed by the fasteningforce of the clamping bolts, so that a required sealing pressure isgenerated along the seal lines SL1 and sl2 to thereby seal oil and thelike.

[0266] At the time of fastening, the compression-deformed amount of thebead is restricted by the thickness-increased portion 16 provided at theperipheral end portion of the base plate around the combustion chamberopening 3, a high surface pressure is generated at thethickness-increased portion 16, which thereby seals a combustion gas athigh temperature and high pressure.

[0267] When no coating is applied to the surface of thethickness-increased portion 16 to provide the gasket at a low price, thethickness-increased portion 16 of the gasket comes into metal-to-metalcontact with the machined surfaces (joint surfaces) of the engine, andconsequently there is a tool-mark irregularity of 3 to 6 microns on themachined surfaces.

[0268] The explosion pressure by engine operation is not aconstantly-applied pressure but a pulsating pressure; therefore, thereis some pressure leakage from the thickness-increased portion 16 to theouter periphery side. However, the pressure is sealed by the bead BD onthe outer side of the thickness-increased portion 16.

[0269] The bead BD according to this embodiment is so structured as togenerate a required sealing pressure by a composite spring of thebase-plate bead 6 and the rubber bead produced when they are compressedand deformed, and this composite structure makes it possible to reducethe hardness of the base plate 2 that forms the base-plate bead 6. Thebeads contact the upper and lower joint surfaces at the flat surfaces ofthe compressed and deformed elastic sealing material 8 and 10, and thesoft elastic sealing material 8 and 10 come into tight contact with thejoint surfaces, eliminating any small spaces in the tool marks, therebysealing the combustion gas that leaks from the thickness-increasedportion 16 under pulsating pressure mentioned above.

[0270] In a gasket of a structure that an elastic sealing material isfilled only in the concave portion of the base-plate bead 6, when theelastic sealing material 10 is compressed and deformed, an externalforce is generated to deform the base-plate bead 6 and the flat portionon each side continuous to the base-plate bead 6 in such a manner thatthey warp upward. The lower the hardness of the base plate 2 is made toinhibit fatigue failure of the base-plate bead 6 and hold down the costof the base plate 2, the more conspicuous the deformation, such asupward warp is likely to become. In this embodiment, the second elasticsealing material 8 is provided also on the convex portion side to letthe second elastic sealing material 8 be deformed to prevent deformationof the base-plate bead 6 and the base plate 2, thereby preventing thedeterioration of the seal performance by the first elastic sealingmaterial 10 in the concave portion.

[0271] In the filling of the first elastic sealing material 10, thecenter portion of it is likely to cave in a little in a transition fromhigh temperature to open cooling. In this embodiment, protrusions 11 and9 are formed, even when the gasket is adopted in an engine whosefastening axial tension is weak, a stable seal performance can besecured at low lost in the region outside the circumference of thecombustion chamber opening 3. After the bolts are fastened, theprotrusions 11 and 9 are in a crushed and flattened state.

[0272] No fastening problem arises in the vicinity of bolts 4 as long asthe bolts are fastened properly. However, the oil holes 5 and the chainchamber hole 17 are in an improperly fastened state because they areremote from clamping bolts. Because the engine is subjected to repeatedthermal cycles as many times as it is used, the fastening axial tensiondecreases to some extent. The gasket is deformed by heat during engineoperation, thus aggravating the sealing condition.

[0273] In order to implement a complete seal under those adverseconditions, in the prior art, in the bead structure having the elasticsealing material filled in the concave portion of the base-plate bead 6,if the hardness of the base plate 2 is increased, the spring force isincreased, but the bead may suffer fatigue failure by vibrationamplitude, and it is not desirable to increase the hardness so much; onthe other hand, if the hardness of the base-plate bead 6 is decreased,deformation mentioned above will occur, resulting in a decrease in thespring force. To make up for this shortcoming, in this embodiment, asdescribed above, in addition to the first elastic sealing material 11filled in the concave portion of the base-plate bead 6, the secondelastic sealing material 8 is formed on the convex portion on thereverse side of the concave portion, and the second elastic sealingmaterial 8, structured such that its width is wider than the width ofthe base-plate bead 6 and its height is substantially the same height ofthe base-plate bead 6, serves to prevent deformation of the base plate 2and the base-plate bead 6.

[0274] As the hardness of the base plate 2 is lowered, the spring forceis made low, but because the first elastic sealing material 8 is formed,on the convex portion, with a height equal to the height of thebase-plate bead 6 to thereby regulate the deformation by the elasticsealing material 10 filled in the concave portion, with the result thatthe BD bead is provided with a spring force equal to or greater than aspring force by a structure that uses the base plate 2 of ahigh-hardness material.

[0275] Further, when the elastic sealing material 10 is formed bymolding, the concave portion side of the base-plate bead 6 is processedso as to be flush with the flat surface of the base plate 2. Duringmolding, the sealing material 10 expands thermally by high temperature,but when it is open-cooled, the central portion of the rubber large inthickness shrinks by an amount corresponding to thermal expansion, andcaves in slightly, and in the portions, away from a clamping bolt, whichare not fastened properly and overhang, the surface pressure maydecrease, leaving chances of pressure leak.

[0276] As countermeasures, according to the invention in this patentapplication, as shown in FIG. 38, a small protrusion 11 is formed in themiddle of the surface of the elastic sealing material 10 in the concaveportion of the base plate 6, the bead is deformed without increasing thefastening load so much, and when the surface pressure decreases, theprotrusion 11 formed on the surface of the first elastic sealingmaterial 10 bulges and deforms accordingly. Though small in terms ofarea, the protrusion generates a high surface pressure, and serves toapply a complete seal. In other words, located in a position away fromthe clamping bolt to the outer circumference side, the surface pressuretends to become relatively small, the elastic sealing material on bothsurfaces of the base-plate bead 6 is normally pressed to the opposedjoint surfaces to apply the seal by the compressed and deformation ofthe bead by the fastening load. At this time, the protrusion 11 formedon the elastic sealing material 10 on the concave portion side isdeformed in a manner to be pushed into the concave portion, adaptsitself to the flat surface of the joint surface, and becomessubstantially flush with the flat surface (underside) of the base plate2.

[0277] When, from the steady state, the clearance between the opposedjoint surfaces at the bead position increases by vibration, for example,the surface pressure temporarily decreases, the compression-deformedamount of the bead decreases, thus reducing the sealing pressure. At theelastic sealing material 10 on the concave portion side, according to anincrease in the clearance, the protrusion 11 automatically bulges tosecurely retain contact with the opposite joint surface and has thecontact surface decreased, and can maintain the seal condition by anincrease in the surface pressure by the protrusion. As the clearancedecreases, the steady state is restored.

[0278] In the foregoing, description has been made of a case where thesurface pressure decreases with changes in the clearance between theopposed joint surfaces. Even when the clearance between the opposedjoint surfaces remain unchanged or even when the spring force decreaseswith deterioration with time and the surface pressure becomes smaller,as described above, because load concentrates on the protrusion 11 asthe surface pressure decreases (the protrusions 11 and 9 do notnecessarily bulge in this case), the surface pressure rises at theposition of the protrusion 11, making it possible to maintain aspecified sealing pressure.

[0279] When the change in the gap at the bead position between the jointsurfaces is large, it is preferable to form a protrusion 9 also on theelastic sealing material on the convex portion of the base-plate bead 6,as shown in FIG. 40.

[0280] The protrusions 11, 9 arranged in the bead-width direction arenot limited to one, and may be two or more as shown in FIGS. 41 to 43.When two or more protrusions are provided, the height of the protrusions11, 9 may be made different. The magnitude of the protrusions may alsobe different. When a plurality of protrusions are formed, load ofsurface pressure can be alleviated, or if the surface pressuredecreases, a labyrinth effect may be obtained by the plurality ofprotrusions 11, 9 or the de facto increase of the seal lines SL providesan effect of a stable sealing property for an extended period of time.

[0281] Further, with regard to the plurality of protrusions 11, 9, bymaking variations in the size or shape (the area unit length in alongitudinal sectional profile or a plan view) of the protrusions 11, 9to seek optimization of the protrusions 11, 8 as shown in FIGS. 44 to47, it is possible to enlarge the above-mentioned effects. In otherwords, when providing two or more protrusions in parallel widthwise, itis preferable to make the height of the protrusions 11, 9 relatively lowor reduce the area per unit length on the higher surface pressure side.

[0282] With regard to a single-line protrusion 11, 9 extending along theseal line, it is possible to change the height or shape of theprotrusion 11, 9 according to the surface pressure at the location ofthe protrusion 11, 9. In other words, in the areas where the surfacepressure is relatively smaller, the height and the width of theprotrusion 11, 9 may be increased.

[0283] The protrusions 11, 9 may be formed continuously along the wholelength of the seal lines SL1 and SL2 or intermittently at specifiedintervals.

[0284] When the protrusions 11, 9 are formed partly on the seal linesSL1, SL2, they should be formed at positions that are far from the bolthole 4 and at relatively low surface pressure or at parts where changesin the clearance between the opposed joint surfaces are relatively large(the amplitude of surface pressure change is relatively large).

[0285] In this embodiment, description has been made of the base-platebead 6 as a full bead, but this embodiment is applicable when thebase-plate bead 6 is a half bead in a stepped structure. Morespecifically, as shown in FIGS. 48 and 49, a second elastic sealingmaterial 12 is fixed to the convex portion (the portion rising from theflat part of the base plate) of the base-plate bead 6 in a stepped form,a first elastic sealing material 14 is applied to the concave portion onthe reverse side of the convex portion, and then protrusions 13, 15 areformed at the thick portions. The operation and the effect are the sameas in the above-mentioned embodiment.

[0286] In the bead BD in the vicinity of the thickness-increased portion16, the second elastic sealing material need not necessarily be attachedto both sloped sides of the convex portion as shown in FIG. 50. In otherwords, receiving a relatively high surface pressure and having a strongforce to constrain the base plate, the thickness-increased portion 16inhibits the base plate from being deformed. This applies to theportions in the vicinity of clamping bolts.

[0287] The height of the protrusions 11, 9 should be designed such thatthe deformation ratio is not more than 25% when the protrusions aredeformed to reach the thickness of the thickness-increased portion 16,regardless of the shape of protrusions.

[0288] In the above example, description has been made of a metallicgasket having a single base plate. In a metallic gasket, a plurality ofbase plates, each having the above-mentioned structure, may be stackedone over another according to the space between the joint surfaces. Inthis case, the base plates need not necessarily be stacked such that theconvex portions of the base-plate beads are arranged face-to-face witheach other as in prior art.

[0289] The other aspects of the structure, the operation and the effectare the same as in the above-mentioned embodiments.

[0290] Industrial Applicability

[0291] As is clear from the above description, in the present invention,since the amount of elastic sealing material on the convex portion sideof the metal bead is increased, the compression-deformed amount isincreased, with the result that the elastic sealing material can beprocessed more easily. In addition, because the thickness of the elasticsealing material can be increased, a larger processing tolerance can beset, which makes it possible to reduce manufacturing cost.

[0292] Further, the elastic sealing material fixed to the convex portionof the metal bead is less likely to be exposed to cooling water, and theelastic sealing material filled in the concave portion of the metal beadis covered by the bead and is not exposed to cooling water. Therefore,the elastic sealing material parts are prevented from deteriorating,making it possible to maintain the stable seal performance for extendedperiods of time.

[0293] Further, a necessary sealing pressure can be obtained by synergyof the resilience of the metal bead and the elastic resilience of theelastic sealing material fixed to the convex portion and filled in theconcave portion. The resulting effects are that the hardness of the baseplate material can be reduced, thus eliminating need to worry aboutfatigue failure of the metal bead of the base plate, and that theroughness of the seal area is absorbed, making it possible to properlyseal cooling water, oil pressure, or the like with a lesser surfacepressure without increasing the bead width to an inappropriate extent.

[0294] Further, wide seal areas can be obtained for the elastic sealingmaterial parts on the convex portion and in the concave portion of themetal bead, and the consequent results are that the flaws on the jointsurface and the blowholes that occur in casting can be sealed properlywith a low surface pressure, that the elastic sealing material, arubber-based material above all else, has a smaller gasket factor,making it possible to a limited axial tension load in the areas underunfavorable conditions and thus decrease total load.

1. A metallic gasket including a base plate made of a thin metal plate,said base plate having one or not less than two openings, such as boltholes, and having beads formed along seal lines and a firstthickness-increased portion made thicker than the remaining portions ofsaid base plate, a surface pressure being made to concentrate on saidfirst thickness-increased portion and said beads being deformed in athrough-thickness direction to thereby seal a joint of opposing jointsurfaces when said metallic gasket is sandwiched between the jointsurfaces and fastened by clamping bolts, wherein said bead comprises, incombination, a metal bead and a rubber bead, said metal bead beingformed by bending said base plate in a through-thickness direction tocreate a convex portion, on one surface side of said base plate, with aheight higher than the thickness of said first thickness-increasedportion, and said rubber bead made of an elastic sealing material beingfixed to a surface of said convex portion of said metal bead and filledin a concave portion opposite said convex portion, wherein said rubberbead is compressed and deformed in the through-thickness direction incooperation with deformation of said metal bead.
 2. A metallic gasketaccording to claim 1, wherein said metal bead is either a full bead or ahalf bead in a stepped form.
 3. A metallic gasket according to claim 1,wherein said elastic sealing material fixed to the surface on saidconvex side of said metal bead is fixed at least to the surface of saidconvex portion.
 4. A metallic gasket according to claim 1, wherein theheight of said elastic sealing material fixed to the surface of saidconvex portion of said metal bead is equal to or substantially equal tosaid convex portion of said metal bead.
 5. A metallic gasket accordingto claim 4, wherein some of openings formed in said base plate arecombustion chamber openings and that said first thickness-increasedportion is formed in a manner to endlessly encircle said combustionchamber openings.
 6. A metallic gasket according to claim 5, wherein aperipheral end portion on said combustion chamber opening side of saidbase plate is folded back and said first thickness-increased portion isformed by putting a shim plate inside a folded-back portion.
 7. Ametallic gasket according to claim 5, wherein a total thickness of saidfirst thickness-increased portion is made thin in the vicinity of holesfor receiving clamping bolts and is made thick between saidbolt-receiving boles and said total thickness is varied in thecircumferential direction of said combustion chamber openings.
 8. Ametallic gasket according to claim 5, wherein said secondthickness-increased portion thinner than said first thickness-increasedportion is provided in a region along the outer periphery of said baseplate.
 9. A metallic gasket according to one of claims 1 to 8, whereinlubricant is applied to one or both surfaces of said base plate.
 10. Ametallic gasket comprising a base plate made of a thin metal platehaving formed therein a plurality of combustion chamber openingsarranged adjacent to each other, and a thickness-increased portionthicker than the remaining portions of said base plate, and seal linesformed encircling said internal circumference of each combustion chamberopening, and beads formed along said seal lines, wherein in a boundaryarea between said adjacent combustion chamber openings, common beads areformed and shared by beads around said adjacent combustion chamberopenings, wherein beads formed along neighboring peripheral edges ofsaid adjacent combustion chamber openings are combined to form a bead inan integral structure, wherein the width of said thickness-increasedportions located in said boundary area and extending along said internalcircumferences of said combustion chamber openings are set in proportionto a clearance between said adjacent combustion chamber openings,wherein said bead is a composite body of a metal bead formed as a convexportion having a height higher than said thickness-increased portion bybending said base plate in the through-thickness direction, and a rubberbead formed by fixing to said convex portion surface of said metal beadan elastic sealing material capable of exhibiting a spring force bycompressing and deforming in the through-thickness direction and byfilling said concave portion on the reverse side of said convex portionwith said elastic sealing material, and wherein said elastic sealingmaterial is fixed at least to the surface of said convex portion of saidmetal bead and the height of said elastic sealing material is set to beequal to or substantially equal to the height of said metal bead.
 11. Ametallic gasket according to claim 10, wherein said thickness-increasedportion is varied in thickness partly to equalize the surface pressurewhen said gasket is inserted between said joint surfaces.
 12. A metallicgasket according to one of claims 1 to 8, 10, and 11, wherein at least aprotrusion height or a width of said metal bead is varied partly alongsaid seal line to thereby equalize said sealing surface pressure by saidbead provided along said seal line.
 13. A metallic gasket according toone of claims 1 to 8, 10, and 11, wherein at least a protrusion heightor a width of said rubber bead is varied partly along said seal line tothereby equalize said sealing surface pressure by said bead providedalong said seal line.
 14. A metallic gasket including a base plate madeof a thin metal plate, said base plate having a plurality of seal lines,a bead formed along each of said seal lines, and a converge-diverge partformed where at least some of said plurality of seal lines converge ordiverge at specified points, wherein said bead formed along each of saidseal lines comprises, in combination, a metal bead and a rubber bead,said metal bead being formed in a convex form only on one surface ofsaid base plate by bending in a through-thickness direction thereof anda rubber bead made of an elastic sealing material being fixed to asurface of said convex portion of said metal bead and filled in aconcave portion on the reverse side of said convex portion, said rubberbead being compressed and deformed in said through-thickness directionin cooperation with the deformation of said metal bead, and wherein atleast in said converge-diverge part, said elastic sealing material on asurface of said convex portion is fixed at least to a surface of saidmetal bead and the height of said rubber bead is set to be equal to orsubstantially equal to the height of said metal bead.
 15. A metallicgasket according to claim 14, wherein a thickness-increased portion isformed on said base plate by partly increasing the thickness of the baseplate in the through-thickness direction, and the compression-deformedamount of said bead is regulated by an increased thickness of saidthickness-increased portion.
 16. A metallic gasket according to claim14, wherein by adjusting the width of said rubber bead in saidconverge-diverge part, said sealing pressure in said converge-divergepart is made to match or come close to said sealing pressure at saidseal lines other than said converge-diverge part.
 17. A metallic gasketaccording to one of claims 14 to 16, wherein in said converge-divergepart where said metal bead diverges from a full bead into a plurality offull beads or a plurality of full beads converge into a full bead, saidrubber bead width is adjusted so that the ratio of said rubber beadwidth on said convex portion side of said metal bead to said metal beadwidth becomes smaller in said converge-diverge part than in theremaining portions of said bead.
 18. A metallic gasket according toclaim 14, wherein in said converge-diverge part where said metal beaddiverges from a full bead into a plurality of half beads in a steppedform or a plurality of half beads in a stepped form converge into a fullbead, said rubber bead width is adjusted so that in the vicinity of saidclamping bolts the ratio of said rubber bead width to said metal beadwidth becomes larger in said converge-diverge part than in the remainingportions of said bead.
 19. A metallic gasket according to one of claims1 to 8, 10, 11, 14 to 16, and 18, wherein one or not less than two linesof said protrusions are formed along said seal line on at least one ofthe surface of said elastic sealing material fixed to the surface ofsaid convex portion and the surface of said elastic sealing materialfilled in said concave portion, said elastic sealing material partsconstituting said rubber bead.
 20. A metallic gasket according to one ofclaims 1 to 8, 10, 11, 14 to 16, and 18, wherein one or not less thantwo lines of protrusion are formed where said sealing surface pressureis relatively low on at least one of the surface of said elastic sealingmaterial fixed to said convex portion and the surface of said elasticsealing material filled in said concave portion, said elastic sealingmaterial parts constituting said rubber bead.
 21. A metallic gasketaccording to claim 19, wherein at least one of the height and the widthof each line of said protrusion is varied in the extending directionthereof according to said sealing surface pressure at a position wheresaid protrusion is formed and a larger value of at least one of theheight and the width of said protrusion is set where said sealingsurface pressure is lower.
 22. A metallic gasket according to claim 19,wherein a plurality of protrusions are formed on at least one of thesurface of said elastic sealing material fixed to the surface of saidconvex portion side and the surface of said elastic sealing materialfilled in said concave portion, and in said plurality of lines ofprotrusions, at least one of the height of said protrusion and an areaper unit length is varied according to the sealing surface pressure atthe position where said protrusion is formed.
 23. A metallic gasketaccording to one of claims 19, 21 and 22, wherein a plurality of baseplates are stacked in a multilayered structure.